It all depends on what type motorhome you are considering. Older gas-powered motorhomes had carburators rather than fuel injection, the 4L80 transmission in early Chevrolet chassis was worthless--later coaches used Allison transmissions. Chassis were made heavier-duty etc.
High-line diesel pushers like ours and similar units will probably go forever if cared for.
Carburators and 3-speed transmissions kept mileage low--4- and 5-speed transmissions with lock-up and fuel injection improved mileage in the early 90's. My dad's '89 Pace Arrow got 6.5 MPG on the same roads our '91 Pace Arrow got 8 MPG. Don't look for much improvement in the mileage of newer gas motorhomes. With DPs, bigger engines and emission controls have adversely impacted mileage in newer DPs. Weight with slides of course had a big impact. Late versions of our coach probably get around 2 MPG less than our coach, but they are heavier and have more HP.
Until you get more specific on what you are looking at, it is hard to give generalities.
Good luck!
