I would guess 6 to 10 MPG.
IMO, if you are truly concerned about fuel economy, you should probably not buy a motorhome of any kind. In fact, I do not believe "motorhome" and "fuel economy" should ever be used in the same sentence (even though I just did it. Twice!)!
I have had my coach for several years, and have logged every fuel stop in a book. I have never bothered to calculate the "fuel economy". IMO, it is an exercise in futility. I know it is atrocious, I don't need to prove it!
That doesn't stop me from driving the thing!
The floor plan is, IMO, much more important than any mileage concern. That is based on experience. I once had a Class A with a terrible floor plan. We lived in it for a while, but soon traded it for a rear bedroom unit. Much better!
Even though we are very happy with our gas couch, there are times when I wish I had looked for a diesel. Especially now that diesel fuel is about 6 cents cheaper than 87 octane unleaded!