Last summer we drove the RV 6,198 miles towing our 2006 CR-V. Sometimes when I filled the fuel, I just put in a fixed $$ amount and didn't get the tank full, but at the end of the journey I filled it completely, just like it was when I left the fuel station at home at the beginning of the trip. Total fuel mileage for the entire trip was 9.0944 mpg. This is a 2000 Itasca Horizon, 36 foot, 300 HP Cat diesel.
On this trip, we drove northwest from Texas to Alberta, west on the Crows Nest Highway across the Rockies and Cascades to Vancouver, south on the Washington/Oregon/California coast road, then east back across the Cascades and Rockies, then southeast from Kansas to north Texas. 9.0944 mpg, calculated by Microsoft Excel. It surprised me that the mileage was not greatly different across the mountains than in the less hilly country.
I don't drive fast; usually try to keep it about 56 - 57 mph, just fast enough to stay in 6th gear.
