I have always said if you worry very much about mpg then think twice about having an RV. Fuel costs is a small item in terms of buying and operating an RV.
Some exceptions might be for folks who live full time in their RVs in order to save money and tend to stay put in one place for months at a time.
While we are RVing we tend to average just about the same total cost as when we are at home. We stay at a mix private, public campgrounds and other free overnight parking. This is not say I won't make some effort, not very much, to find lower priced fuel if I can. But even then easy in and out may out weight difference in fuel costs.
Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & Coach House 272XL E450
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB