I have a
little experience towing travel trailers. I won’t tell you what to do, but I will try to give you a better picture on what’s going on and you can decide which way to go with it.
The truck:
1997 5.7 Vortec automatic 4x4. Factory rating was 255 hp and 330 LbFt. While that sounds pretty good on the first look, what is important is where that measurement was taken – 255 @ 4600 RPM, 330 @2800 rpm
at the crank. This will become important a little further down.
3:42 gearing. Great for MPG (relatively), not so great for towing. 3:42 gears put you pretty low in the torque curve which hurts you when towing, especially if you’re not “gearing down”. What this does is make the truck a “dog” in anything other than just running from here to there. If you’re not running up around 2800 rpm, you’re not getting 330 lbFt of torque. As you drop off that 2800 rpm, gassers (as opposed to diesels) tend to drop off quickly in the torque curve. If you’re running 1800-2000 rpm, you could easily be down to ~250 lbFt. That’s not a lot for a combined weight of around 9,000-10,000 lbs. See the next paragraph for how it even gets a little worse.
Keep in mind that those OEM ratings are at the crank, not where the rubber hits the road. Available force at the road surface is what matters, since this is what is propelling you forward. OEM’s quote crank because it makes for better ad hype. What really matters is how much gets input to the tire to road point. Typical drivetrain losses are somewhere in the 20-25% range with an automatic drivetrain. You’re 4x4, so you can toss in another 5 %. So let’s say 25% drivetrain losses. Assuming your engine is in tip top shape and making exactly what the ratings are (seldom ever happens, they don't use any old engine off the production line to establish the ratings), now you’re talking 198 RWHP @ 4600 and 247 RWTQ @ 2800 Rpm. If you're runing around 2000-2200 Rpm at highway speeds because of those 3:42's, it's entirely possible you're only putting something like 150-160 hp and 190-200 Tq to the pavement. You can make 200 LbFt yourself with a torque wrench, this is what you are using to drag a 9,000-10,000 combined weight. Yipes!
For comparison, my 98 K2500 6.5 TD runs ~275 RWHP @ 2800 rpm and 450 RWTQ @ 2100 rpm (It's not stock). My 4:10 gears put me right at 2200 rpm at 62 Mph (100Kph). With our 8,500-9,000 lb TT I can run 50-60 mph up mountain passed like 4th of july, lookout, Vantage highway, etc. But it’s working hard (210 ECT and 1150 EGT, even with the water meth injection spraying) and I usually only run the 45-50 that traffic usually ends up at anyways. Even if I dropped to 5000 lbs it would still be a lot of hard work, I couldn’t imaging trying to do anything like that with around 250 LbFt at the crank/198 at the wheels!
3:42 gears also mean the truck likely does not have a “tow package”. Meaning wiring, brakes, cooling, etc. This means a lot of extra wear on the vehicle when towing.
IIRC, 3:42 trucks were rated at 5000 lbs towing. While your trailer strictly falls under that weight, you’re probably closer to the max your truck can deal with than you think. More on that further down.
You’ve mentioned a few things that leads me to believe you’re savvy enough to have checked for stored/active/pending trouble codes so you can probably rule out vehicle issues as a cause. Especially if it runs fine unloaded. Timing, tune up, filters, synthetic oils and such things will grab you another 5-10 hp at best (assuming there was something wrong to start with). All respect to Michael (who is pretty knowledgeable), a 97 should throw a converter efficiency code if the catalytic convertors are “plugged”. That’s why they have an upstream and downstream O2 sensor. It is a possibility, but an outside one in my opinion. A larger exhaust
may help you make a few more ponies, but beware of where it moves the torque curve. You may end up worse (when towing) than you are now. You can change it out if you want, but I think it's a bit of a "red herring"....
The trailer:
I looked up the Starcraft site and your model (as best I could figure).
You say 3900 loaded with gear, so it’s reasonable to assume you’ve had it on a scale. If not, you should. RV manufacturer weight ratings are more often than not wildly in error. The reason being they weight the base trailer so they can get a low weight (for sales hype) and then add options. Some manufactures consider things like a fridge, stove or even furniture "options". The never include things like propane tanks or batteries. My trailer was factory weighted without the slides installed. Suffice to say, it's a bit heavier on the actual scales (empty) than the placard on the side. If you haven't had it on the scales, you could possibly be dealing with a 4500 lb trailer instead of a 3900 lb. Just find a truck stop on the interstate that has the "certified Cat scales" or something similar. They don't care what they weigh as long as they get paid.
Your models also appears to be just over 7.6 feet wide and 8.6 feet high (not including the AC unit). That’s nearly as much frontal area as my 35’ 2011 Forest river 831RLBSS (~9500 lbs loaded).
Where this becomes important is aerodynamic drag. This is something that people usually overlook when matching trailers to trucks. It’s not just the weight ratings that matter.
There is no comparison between hauling a travel trailer that weighs 3900 lbs and a flat deck loaded with scrap that weighs 3900 lbs. The aerodynamic drag is going to make the truck “see” a much much heavier trailer. So while your truck may be able to pull a loaded flatdeck relatively easily, a travel trailer is going to make it scream for mercy. Try standing in the bed of you truck and hold a 4x8 sheet of plywood in the breeze at 10 Mph. Impossible. Not make it a 7.6x8 foot sheet and think of it at 65 Mph. That’s what’s behind your truck… and that’s just DRAG, we haven’t even considered the weight, extra rolling friction from the trailer tires, attitude change of the truck from tongue weight, etc, etc, etc…
The length of the trailer is also important. It is the side of the trailer, but the sides also impart drag to the whole rig. 18 feet isn’t a lot compared to other trailers, but it’s not exactly small either. That 18 feet is significant. If there’s a cross wind component of any sort, that 18 foot slab side starts to become part of the frontal drag. Not good. Even with a straight headwind, the sides still impart a drag component.
Even with a straight headwind, drag is worse: a 5 mph headwind makes 65 Mph forward speed become 70 Mph. Drag is exponential. While it make take 5 HP to go from 50-55 mph, a 55-60 mph jump may require another 10 or 15 hp…..
Drag also become significant at the speeds you talking, even running down hill. Remember: Drag is exponential. It takes a sharp rise right around 50-55 mph for vehicles. That’s why you see statements like” it’s better to roll up the windows and run AC at highway speeds.” At 65-70 mph, drag is even higher. Toss in that great big 7.6x8 foot sail behind you and it’s even worse. That means more power required to attain and maintain the speed, which means more throttle, which means more RPM, which means more fuel. And those 3:42 gears are keeping you well below the engine torque peak.
You can’t just look at the trailer weight, the trucks rating and then expect it to be good to go. There’s more at play in the equation than that. Unfortunately, most people don't know these things when the buy a trailer. The newer vehicles can get away with it a bit easier than the "Vortec era" vehicles because (comparatively) they tend to be overpowered everywhere. God Bless the HP wars.....
The altitude:
The greatest change in the atmosphere happens below 10,000 feet. The thing that matters in this discussion is pressure. For example - the change in atmospheric pressure between 0-5,000 feet is far greater than 10,000-15,000. Less pressure means less cylinder filling (NA engines are dependent of atmospheric pressure for cylinder filling), which means less available mass of oxygen to burn with the fuel and produce power. The percent O2 remains the same, but there is less of it per volume as the pressure drops.
6500 ft ASL on a naturally aspirated engine is going to suck power out of it. This means your truck can be down a significant amount of power. The MAF sees less air mass passing though the engine and starts tailoring back the fuel . less fuel and less O2 means less power available.This means more throttle, more rpm, more fuel, etc just to maintain power (if it even can). This is why turbochargers are favored on diesels: the forced induction is far less effected by altitude (there are other reasons for turbos on diesels, but not relevant for this discussion). Much like those semi’s you mention passing you. FYI, I wouldn't try to use a rig as a comparison. They have ~16 liter engines, 1000 lbft tq, 16-20+ fwd speeds, gear splitters, sequential turbos, intercoolers and can be running anywhere from empty to 80,000 GVW. Those same semi’s are also ecstatic if they get more than 6-7 mpg on the flat....of course, when you run 1,000,000 miles a year, 2 or 3 mpg difference becomes a pretty significant dollar figure.
You also get a double whammy from the altitude when towing: less engine power due to less O2 mass, but the drag reduction is not proportional to the power loss.
This is why running downhill often requires throttle: aerodynamic drag is greater than the accelerative force of gravity. If you’re not talking a steep grade, you need even more throttle since gravity is contributing even less the vehicle's forward motion.
MPG:
see the running theme of aerodynamics in the previous paragraphs. Factory Mpg ratings on a gasser 5.7 run in the 15 city/18 highway range. It’s important to note that the 18 highway rating is around the 55 mph mark. Run it faster and it starts dropping pretty quick. Increase the aerodynamic drag and it starts to drop like a stone. 5 mpg is pretty low, but 8-10 mpg towing on a 5.7 at 65 mph wouldn’t be out of the ball park. My diesel gets around 13-15 towing our 35 footer, but that a whole different ballgame.
The dealer:
Dealers don’t really know much about what they sell. RV dealers are probably the worst of the bunch. They will make whatever fits their needs in order to make the sale. They generally fall into three categories for me: those that think they know what they are talking about, those that don’t know what they are talking about and those that lie by omission or intentionally. So while they may say your truck will haul XX fine, they really don’t know. They don’t because unless they look at the specifics of your truck, they don’t know what it is capable of. “1/2 ton towable” is just a sticker on the side of the trailer, it means nothing except that they can catch a few poor unknowing souls. Just because it’s a truck doesn’t mean it can do everything. Many properly equipped SUV’s or minivans can haul 5,000 lbs but you don’t see “Minivan towable” plastered on the sides of trailers. What I’m getting at is: whatever they told you, forget it. That and a quarter will get you a phone call….
The exception to the rule here is the F150 ecoboost trucks with the 10,000 lb max tow package and things like 1500HD models. Truthfully, those aren’t really half tons anymore either...
Summary:
So, at the base of all this, what I’m getting at is your truck is a little under-equipped for the trailer you’re towing. Not a lot, but it is under for your intended use.
While I haven’t seen your particular truck, a 3:42 tends to tell me the truck’s intended use (by the OEM) is passenger car like duties and light yard work.
It’s a combination of gearing, cooling, and a few other things that are your "thorn". If I were to give you my opinion (and only my opinion), I would say gear it down a bit (3:73, but 4:10 would probably not be to your liking when not towing) and add some aux cooling like a big transmission cooler and maybe and engine oil cooler. Or, drop the transmission down a couple gears and get used to driving at slower speeds and or getting passed a lot.
No matter which way you go, I would recommend a transmission cooler. The 4L60E is a marginal transmission in a truck application at best. Adding towing duties on will work it that much harder. Which means heat. Heat kills transmissions in short order. You want to make sure it can shed that heat or it will lead a short life. Just an FYI: finned or “tubed” transmission pans have been shown to be pretty much ineffective at any significant drop in trans temps. You need a proper cooler.
Either way, it’s never going to be a towing king. But you can make it a little better for what you are using it for.
Good luck.
