Our RV is what some call a Class B+. it is 26.5' long. It doesn't have the overhead bed, instead there is a TV and electronics up there. The overhead is much more aerodynamic than a Class C with a bed up there. It is also 6" narrower than most Class Cs and the transition from cab to house is slanted rather than at right angles to the cab. All of this contributes to a more aerodynamic design. I also try to stay around 58-60 MPH when travelling. Ours is on a 2008 model year E450 chassis with the V10. When we were test driving different RVs, I did find that RVs like ours were easier to drive and less affected by wind and large trucks.
Even with all that going for it, I average 9-10 MPG. I do not tow anything. My advice is not to concern yourself so much about MPGs and concentrate more on getting a rig that meets your family's requirements.