Our 32' Class C with 2 slides on the Ford V10 gets about 7.5 - 8.5 mpg on the highway. I usually drive within a mile or two of the speed limit (on the highway around here is 70 mph). Driving in the hills/mountains of Georgia, North and South Carolina, we get about 6.5 mpg. With mostly city/local driving we usually average 4.5 mpg.
I did not notice a difference in gas mileage between mile 1000 and mile 30,000. It also doesn't seem to matter if we are running heavy or running empty. The biggest impact on mileage for us is the type of gas we use and speed. We get the best gas mileage with BP, Sunoco, Exxon, & Mobil. We get the worst gas mileage from Flying J and Loves. We see the best gas mileage in the 60 - 66 mph range, decent mileage from 67 - 74 mph and poor mileage over 75 mph.
-Michael