The end all thread on class A gas mpg with v10

I have had two F53 chassis both bought new, one prior to 1998 and the other after 2001, two different horsepower ratings. Neither V-10 got over 8mpg even going down hill, I believe the average on motorhome is 7-7.5mpg with a clean air filter and fuel filter. I have friends with the V-10 in a f350 pickup truck pulling nothing still gets around 8mpg at best. Anyone getting over maybe 8.5mpg on occasion with a motorhome needs a new calculator.