Just got back from a 13,000 mile trip to Alaska and back in a "C" with a Ford and a 460 engine. Not going over 65 mph I averaged 9.2 mph but much of that driving was much slower. I lose one mile per gallon running 70 mph rather than 60-65 mph. We had a diesel pusher and averaged 9 mpg. Talking to a guy one time he shook his head and said he didn't see how I could afford to drive it across country. I told him my yearly insurance and maintenance were much more than my annual fuel bill because I didnt drive the RV the thousands of miles a year I do my cars. Pencil out the difference per year between getting 7 mpg and 9 mpg. It is negligible compared to what you have spent for the rig. Buy the size you want and enjoy.