I've used a 98' jeep grand cherokee with a 5.2, and a 03' Ram with a 4.7 to haul our 2 previous TTs, but when the time came to step to our full-time, family 5er (Montana 3750FL) I wanted something with some power, that was more than enough for the load.
I searched long and hard, and quizzed the guys I work with (who all have big 5ers, and haul ALOT). The things I kept hearing was "duals", and "allison". Since I get free fuel sometimes, and get travel pay, I wasn't worried about mileage enough to let a couple extra MPGs cost me nearly $10k. So I found a nice 2001 3500 crew cab dually with an 8.1l vortec, and a 5 speed allison. I hooked it up to my 39'1" Montana the other day, and it barely made it grunt. I love this truck.
It had a gooseneck hidden ball hitch when I got it, so I got the 20,000# companion gooseneck to fifth wheel hitch for it. I also added a tool box, and a 100 gallon transfer tank. I have to switch the tool box and the tank soon (the corner of the trailer will hit the fuel pump in the tank if I turn past 80 degrees or so. Otherwise, I wouldn't change a thing.