I have a 2000 K3500 CCSB Dually and 7.4L engine. Since its the standard bed, I have a 25 Gallon fuel tank. So far this is the biggest gripe about my truck (along with the 6.5ft bed). I thought about an in bed tank but we would lose quite a bit of storage, and most of them are rather expensive. We took this truck on some long-ish distance camping trips and my horrific fuel mileage (working on some issues with that) and thimble for a fuel tank cause me to stop and fill up about every 150 miles to stay safe. I have seen some of these 1 Tons came with auxiliary tanks, but I have not found one to purchase or any other information about it. Other than you can buy a tank sender and lift pump for them on rock auto. I thought about adding either a suburban or tahoe tank between the rear frame rails. I don't have a spare tire so perhaps that would be a good place to put a tank. I would either need to weld bungs on both tanks and run a large hose between the two tanks and make essentially one large tank but that would require a breather of some sort, or plumb in a filler neck to somewhere and sender unit and fuel pump with a fuel tank switch and relay. It would be nice to add another 150 miles to my fuel capacity so if we ever decide to do a long haul trip we aren't stopping ever 2 hours for fuel.