I just completed a 6000 mile trip through 13 eastern states from Florida and back and got 18 mpg until i got to West Virginia, where it dropped to 17mpg.
I drove 60mph and have 60lps of air in front tires and 75lps in the rear. Cruse control was used almost all of the time.
The on board Onan 2.8 generator was used about 8hrs from the same gasoline.
This trip included both interstate, regular roads, and city driving.
I was happy to get more than 17mpg for combination highway and city mileage.