Here in southern California, there are a lot of grades traveling north along the coast. We have often gotten as low as 7 miles to the gallon with our 27 Foot long E-450 powered Class C. On flat terrain, driving at a steady 55-60mph we have gotten 9-10 mpg. Some claim better mpg. With a motorhome rolling resistance due to weight, tire pressure, and the aerodynamic drag from total lack of streamlining affects mpg greatly. The faster you go, the more mpg suffers. Best to relax about mpg , eat lunch at rest stops, and minimize eating at restaurants along the way.