The only variable I've noticed that makes much difference is speed -- beyond obvious headwind/tailwind or going uphill/downhill.
I've driven various Class Cs from 24 to 30 feet, and all got around the same 8 MPG. They were all about the same width, same height, same shape, same Ford V10, similar gearing, thus same results.
Weight should make a little difference, but aerodynamic drag, affected by speed, makes most of the difference we can easily control. I've gotten 10 MPG a couple of times on scenic roads driving 45 to 55 MPH, but for most part normal driving in 65 MPH range yielded about 8 MPG on average. Best long trip was 8.6 to 8.7 MPG over 5,000 miles.
Good news is there is lots of room for improvement.