When we towed our 1989 Avion TT 32' with our 2001 Chevy Suburban 2500 2WD 8.1L we had a combined weight of 14,000 lbs. and traveling at 65-70 we used to average 9 mpg. Avion's were more aerodynamic that most of the box type TT's.
By comparison my MH 38' weighing 29K with a 350HP Cummins has averaged 8.234 mpg over a total miles of 45,000, avg. speed of 62-63 mph. This includes when we towed my wife's 96 Toyota RAV4 on a dolly, now our 2006 Honda Pilot (wt.4331 lbs.) 4 down and occasionally my 63 Pontiac Grand Prix in an enclosed car trailer approx. 6K. Plus calculating running my 8K Onan Gen for a total of 684 hrs. If you minus the fuel for the gen at approx. .75 gal. an hour, that calculates approx. 513 gal of fuel and raises the mpg to approx. 9.265 for the MH alone traveling.
So by comparison considering my MH is like a big box going down the highway giving me very respectable fuel mileage. As to current fuel prices I paid $4.069 per gal. for diesel fuel last Friday. If you cannot afford the fuel stay in a motel and drive a Prius.