I've averaged 8.3 MPG over 30,000 miles, the past 14 years. Individual fills (kept track of all of them) ranged from just over 6 MPG to better than 10 MPG, mostly depending on wind and road conditions. I don't know how that compares with your Class A, since you don't say what MPG you got.
If you downsize to a chassis with a small (2.4-3.0 liter) turbo-diesel, you can expect better mileage. Only you will know whether the space is livable. I can tell you a 24-foot C is going to be a quite different living experience from a 37-foot A, almost like going from a 2000-3000 sq ft house to a studio apartment, in terms of the "private" spaces you will no longer have. 24-foot is living in one room.