Let me use my MH as an example. It weighs ~33,000 ready to travel, with a 6k towed I'm rolling at about 39000lbs. I have an ISC 350 with 1050ftlbs. It is coupled to an Allison MD-3066 with 4.88 gears and 22.5 inch tires. I get 8.3 MPG without the toad and first trip with says about 7.8mpg.
Here is the allson brochure that gives the specs on the tranny. Notice input shaft rpm limits. I run 1750 at 67mph in 6th with about 12psi boost, level terrain. At Eisenhower tunnel the grade is about 9% and at 11000 ft. I'm doing 28 to 32 MPH without the towed. While well powered it's no hotrod and as DP go it's a middleweight.
I did the numbers on a PT6 for fun once. It was pretty cool except for fuel consumption, gallons to the mile as I recall. You can make anything work if your wallet is big enough.
I gave up on gas for towing in 1993 with my 6.5TD. If gas makes you happy, enjoy!