Jim:
Our first RV was an '85 Class C (26') with that 460 motor with a 4-barrel carb and enough smog gear for a small town. It was a gas-hog you wouldn't believe. About 6 mpg at sea-level and 4.2 or so at altitude. We have quite a lot of altitude around here. Our northern-most pass over the Cascades (US-20) has a max altitude of 5200 feet and Eastern WA was a favorite place to go. US-20 (the spur to the San Juan ferry terminal) comes within a half mile of our home, so it was our crossing of choice most times.
We gave up on the "C" after the first season, then 3 years or so later we got an '02 Class A (32') with the V-10 - almost 9 mpg independent of altitude. We sold that after 4 years and retired from the lifestyle.
