Best towing vehicles for best MPGs

I get 9.9 @ MTOW. (Maximum Takeoff WT)or MGCW of 20,000 lb on a Chevy HD Duramax 6spd Allison and a 31 ft Arctic Fox. That's a load of *%$#*^ for the Colorado. They test them downhill down wind and have them rolling when they start the test. That's liek saying a Duramax will get 100 MPG. Who the hell is going to buy a VW with their lies. Our mileage is measured from N Va to Co, Tx, Ok, Fl, NY, Oh and Pa on different trips totaling about 13,000 miles a year. I've heard claims of 15 to 17 but no one will hook on my trailer and prove it to me.