we have a winnebago aspect 28, F450 w/V10. We drive a little faster I guess than most. We are getting 8-8.5 @ 65mph. Just crossed Wolf Creek pass and averaged 7.3 from Colo Springs to Pagosa Springs. We tow a Hyundai Elantra 2700lbs. We drove 70 MPH across part of Neb and colo and got 8.3 in Denver. We saw insignificant difference by trying to go slow. We got 9.3 in Neb from Chicago.
I also run a 5 Star tuner which seems to have helped a lot. Engine is more civilized over all. Same was true with my F350 diesel, got almost same at 80 as we did trying to go no more than 70, it has a hypertec tuner on it.
I think some of it is the B+ design, less overall drag from the overhead bed etc., slightly rounded sides. Aerodynamics are important. With a slide in camper in my old pickup we stayed at about 7-8 and it was almost the same design as the traditional class c.
However, weight also plays a major factor. Bigger generally means heavier.