Have to disagree. Drove a Ford Excursion 3/4 ton vehicle could get 14 mpg on highway not towing trailer. When towing and it was a very very tall enclosed trailer it dropped to 9 maybe 10 mpg , hilly highway at that .
I drove a V10 E450 15' cube for expediting. Yes, good power but ate the gas. 8 miles to the gallon I think. Yikes. But then I used it for local work hauling heavy metal/ auto parts. Worked out better.