Our 2009, 32' Class C with 2 slides on the E450 with 6.8l V10 gets between 6.5 and 9 mpg. We travel mostly along the east coast from FL to NJ, so not much in the way of hills and usually drive within a few miles of the speed limit, over or under depending on flow. Most the the states we drive through is 70 mph. Driving through the GA mountains and the mountains in SC, we get about 6.5 mpg.
Driving locally, we get about 4mpg.
-Michael