Meanwhile, a year later... time for an update. It took me awhile, but I have reached the same conclusion that several of you mentioned below:
Pre-DEF Duramax, 2005--06. More tow-friendly engine, more tow-friendly transmission.
Personally I would look into getting a diesel. I own two 7.3 diesel fords. They have been impeccably reliable. A good pre DPF Duramax would be a good choice. I don’t endorse Dodge as I had a few for service vehicles and the trucks just don’t hold up from my experience. I’m not trying to start a truck argument just my experience.
By the time you add up all the mods the costs and everything involved...you’re going to be in a lot of money on a vehicle that simply will not keep its value long term or tow like a diesel would.
If you can swing the price, a crew cab duramax would be the best vehicle for the job. The power/torque and efficiency is lightyears better than any gas engine. You'll double your fuel mileage will more than double while towing and you can blast through the mountains at 80mph without breaking a sweat. 10k travel trailer is a light load for the newer duramax trucks.
I am now actively seeking a mid-2000s diesel crew cab pickup... likely a 1-ton dually because I'd like to have the option to upgrade our trailer to a fifth wheel within the next few years. Even if we DON'T do that, I've only heard good things about the towing stability and control of duallys -- even with a bumper pull. Yeah I know they're hard to parallel park and get through restaurant drive-thru's, but that doesn't bother me. I have nationwide searches set up on AutoTrader, CarGuru, TrueCar, Ebay Motors, and regional on Craigslist. I think I've been bitten by the diesel bug! I'm most comfortable with Duramax since I've owned almost all GM vehicles in my life, but Dodge/Cummins is an option too. All the Fords in the era I'm shopping have the 6.0L which I am avoiding like the plague, unless I happen to find one that's been fully bulletproofed.
At some point my Suburban will be for sale, which makes me a little sad but I'm satisfied with the eventual upgrade decision. Plus my mechanic recently said he was sure I'd have no problem selling, as he knew several guys who'd be interested in the rare 7.4L/4.10 Suburban combo.