I have a 99 Dodge with the 5.9 Cummins. Has about 175K miles on it and pulling my 7k trailer I get about 14-15mpg on the hwy, if I keep my speed reasonable and my foot off the pedal. 21-22 is normal empty....down to about 12 mpg if I am climbing mountain passes.
My buddy and I used to go to a lot of shoots with his 5th wheel and Chev with a 6.0L. Averaged 8-10 mpg on the downhill. He bought a new, longer, and heavier 5th wheel and also got the newer Chev with Duramax and Allison tranny. Pulling the longer and heavier trailer, he gets 12-15 mpg and can pull the hills like a friggin car. The Allison may drop a gear or two, but that truck will pull that trailer up and over a mountain like it wasn't there passing cars on the way.
I have been thinking retirement. If I can afford it, I will get a Kodiak or Chev 4500, 5500 with Duramax and Allison. It will get a little less mileage than the 3/4T Chev, but has better turning radius and better brakes. I could also get a sleeper for it. Deadheading trailers and boats to areas close to matches could make my travel pretty cheap.