I have a 94 Toyota, 6 cylinder, and I got 16mpg. on a 500 mile freeway trip from Orange Co. to Sonoma Co.,Cal. in the summer, with near to 100 degree weather, all day. I ran my dash ac, full force, most of the trip. That includes coming over the Grape Vine. I kept it at a steady 55mph, with about 300lbs of humans, and no cargo, just empty. It might have had water in the tanks, I don't know. I just bought it without checking those things. My first Toyota motorhome, was a Sunrader, 18ft. 4 cylinder. I never checked the mpg, but I don't think it got much better, and it had a lot less power, and when I would come to a hill, I would get the speed up as far back as possible, so I wouldn't wind up going 45 by the top of the hill.
Marshie