When I first started towing my first 5th wheel, we let the dealers talk us into getting a camper that was really too much for my truck to handle. At the time I had a 2003 Dodge 1500 (1/2 ton) pickup. Pulling a 27 foot 5th wheel, I averaged about 6.5 miles to the gallon. After 5 pulls, including one to Texas (about 500 miles), my transmission started to go out, plus the truck just wasn't made for towing (no tow package). I traded in the 1500 for a 2500 Diesel. The best trade I ever made. I have plenty of horse power and I get around 15 mpg while towing (23 mpg regular). I also purchased a Bully Dog Programmer. They aren’t cheap, but in the 1 ½ years I have had it, it has paid for itself.
Hind site is 20/20 and I really wish I had gone all the way with a 3500, as my 2500 is a short wheel base and you don't have much of a turning radius. If it weren't for my slider hitch (designed for short wheel base trucks, pulling 5th wheels), I don't think I could get out of the driveway. For someone thinking about getting a 5th wheel, especially one over 25 ft long, I would recommend a dually (3500 Dodge/Chevy, 350 Ford).