Just for an FYI, I pull a 17' hybrid that weighs 3500 lbs loaded with a 5.7L Hemi and I get 9 to 10 mpg.
I used to have a 37' 5er that weighed 12,000 lbs and pulled it with a 6.4L Hemi in a Ram 2500 that got 7-9 mpg. A 5.9L Cummins before that with that same 5er, 9-11 mpg.
Point is, no matter what you trade up to, for a tow vehicle, you're not going to improve mpg a lot. You might get a big upgrade in towing power, but the mpg won't change much.
The tires that come on those trucks will do the job for you without any modifications needed.