We average about 7.5 to 8 mpg in our 32' Class C with 2 slides. We are about as heavy as they come on the E450 chassis. That average is in driving the speed limit or a couple miles over. Most of our travels, the speed limit is 70 mph. In my opinion, the difference in mpg between a 32' vs a 26' is not as significant as the loss of living space the smaller RV offers.
I could get 9 mpg if I drove 65 - 66 mph. At that speed I get almost the same mileage as a 26' Class C. Those folks are saying 9.5 to 10 mpg. The difference in fuel cost across a 1000 miles is less than $25. I'll take the 6 feet of extra living space for $25 extra per 1000 mile trip.
-Michael