There's alot of factors in mpg and Class C's. Just got back from a 2500 mile trip east and average 10.05mpg the entire trip. Was running 65 most of the time. Our rig is fairly short at 11ft with a shorter overhead bunk vs my grandparents Class C. We did a 3000 mile in theirs and average 7.7. Theirs is a 31ft Itsaca with slide out.. much heavier. I think if it was mine I could get 8.5-9 out of it. Maintenance and the amount of junk you haul around is another factor. Keep only what you need in the tanks and dump on a daily basis if you can.