Chevy vs Ford ;-)
We get 8.9 ave: Ford E450, 26ft B+, 60lb front, 70lb rear, recent alignment, synthetic oil, 55 to 60 mph, feather foot, no toad.  
I believe that regular (no ethenol) gas would improve mileage by about 10%.  My previous Chev (3500 series chassis) unit did about 5% better than my current Ford based unit.   
Its weight, rolling resistance, gear ratio, speed, aerodynamics, fuel type, weather, terrain, and power.
Wish they had a hybrid motorhome that could use sun power to improve mpg by 10 to 20%.  Just paint the whole thing in solar panels. 
I would be doing my happy dance if we got 10mpg. Snowman9000..think you have a good unit.  Nice choice of size, features, and fuel economy.