If you are worried about miles per gallon, perhaps a RV is not for you?
Basically I drove my 1997 RV about 5,000 miles a year, and paid about $800 a year in insurance, and more than I want to consider on RV loan payments. . .
So the $500 a year I spent on fuel back in 1997 was insignificant. The $500 I spend to refill the tank in 2008 several times while moving from Oregon to Phoenix AZ, that was significant, before I got a new job.
Basically why worry about the cost to drive it?
It is more the cost to insure it, pay to store it someplace, and use it.
Yes I have seen people get a class C, and when they first took it out, the mileage was pretty good, but declined over the years. Unloading the 300 pounds of extra stuff can improve the mileage a little bit. So can replacing a defective distributor, bad spark plug wires, and other such maintenance items.
Even cleaning the radiator can make a difference in mileage. If the radiator is very dirty, it can keep the fan 'engaged' more than required, and that can take away a lot of engine horsepower to pull extra air through the clogged radiator.
Good luck!
Fred.