All figures below interstate driving at or about 65mph and all with 6.8L Ford gas.
2016 Class C 27 ft = 6-8 mpg towing. Best mileage was 9.2 w/o towing and 30mph tailwind for an entire day.
2016 Class A 38 ft = same as above (I was surprised too...figured a larger coach would be less mpg)
2018 Class C 33 ft = same as above
All numbers hand calculated using actual miles divided by gallons.
Bottom line is 3 different units and nothing under 6 and nothing substantial over 9. Can you get better mpg with a small coach w/ diesel? Sure, but they cost more so I'm not sure it's a real form of savings when you pencil it out.
If gas $$ are that important, you may get better than 9 but you'll have to drive 50-55 mph....and that is a maybe. I used to cringe the first few times I filled up. Now I just consider it the cost of doing business.
If the money spent on gas is a concern, probably should look at another form of fun. RV'ing isn't cheap.
And, I don't need a wind tunnel or a PhD to prove a brick has horrible aerodynamics
