MILEAGE ON CLASS C

No matter what you do to a Class C, it won't make enough of a difference to make you think you're driving a Volkswagon Beetle. If you don't like the mileage you're getting, either stop figuring it out or get out of RVing. There's a price to be paid for the convenience and comfort of having your "stuff" with you and a bed where you know who slept in it last.

Then you can do something to reduce drag.

Like THIS

There's (at least) one more condition that will affect Class C gas mileage:

How much additional stuff have you got portruding on the roof - covers over the vents will reduce gas mileage a bit, solar panels on the roof will reduce gas mileage a bit, a satellite dome will reduce gas mileage a bit, a standard air conditioner (versus the low profile type) will reduce gas mileage a bit, etc..

Anything on the roof that makes the Class C less "streamlined" will reduce gas mileage.