While I have to say the GM 6.0 is a great engine, and the later model GMs very capable, the Ford 6.2 is stronger, especially with the 4.30 gears. With a new F350, you still get the HD better transmission in the gas or diesel version, and the 6.2 will work, up to 15K actual loaded weight.
I agree with above, a 16,500 lb toyhauler is too much to expect from the gassers.
I would suggest if gas is what you want, get the F350 with 4.30 gears. If diesel is your choice, a late model Ram 3500, would be my choice, if going used diesel.
Jerry