My sister and I are about as close to A vs. C as you'll find. We both have 31' Winnebagos. We have a class A, they have a C. Both with Ford V10 2 valve motors. While I can get 8.0 MPG without a headwind and keeping my speed to around 60, they can get 9 even travelling a little faster. We have the same top 3 gear ratios in the transmissions, and similar final drive ratios (accounting for axle ratio and tire diameters). They are good foot shorter than us in height, so they have quite a bit less frontal area. They are also about 6000 pounds lighter, but as mentioned, going down the highway that doesn't really matter.
But, as has also been said, 1 MPG isn't going to save enough to ever worry about it.