I think your gas mileage is much lower than average for a Class C. Most on the Class C forum indicate an average of 8 mpg. The lowest in our 27 foot Class C was 7.5 mpg, driving 70 mph into a strong headwind in southern WY. Our best was 10. Driving a couple times a year between Western PA and Florida through the Appalachian Mts. we get about 9.0 - 9.5 mpg. We usually drive the speed limit and on cruise control. Did you run long distances with your water, black, and gray tanks full? Did you pull a toad? Are you certain that your tow/haul feature (on Ford E450) was not activated? Were you running the generator while driving? Was your weight under the OCCC? Do you have a new engine in the RV? A new engine takes a little time to break in.
While in FL or other parts of the country that are very flat, I would think doing the speed limit and on cruise control, you should see at least 9 mpg or better.
Bob