I have a 1991 Tioga class C, w/ ford 460 and fuel injection, and now in our 10th year with it - we love it.
my best mileage is keeping it under 65mph, and easy on the foot - 9mpg is best I can get.
IF I drive it harder, and up to 75 mph on the perfect freeway conditions, I'll get about 8 mpg.
I would sincerely doubt any claims of a V-8 or V-10 getting double digits without a full-blown Banks system, or comparable.
I keep my new Michelins aired up properly, and service the engine regularly, with frequent oil changes, filters etc.
over 10 mpg on an average should not be expected with a class C.