I would be extremely wary of anything older than about 1990. We started out on our RV experience with a 1985 26'Class C. It turned out to be the biggest gas-hog I've ever got stuck with. We were very lucky to sell it fairly quickly, on consignment with a local dealer.
I found out, after talking to several owners of similar age rigs, that they were at the last stand by most manufacturers to avoid fuel injection chassis. Ours had enough anti-smog devices to get the USS Forrestal through a smog test. We got about 6.5 mpg at sea level and around 4.5 at altitude. It's difficult to travel around the Cascades and Olympics without running into altitude!
We found out we loved the RV lifestyle, but not with gas consumption that bad. After a couple of years to pay off our gas bills, we got back into the lifestyle with a newer 32' Class A with the Ford V-10. With that engine, gas mileage didn't change with altitude and averaged out at about 8 mpg. Unfortunately, gas was selling at around $4.50 a gallon in the rural areas of Oregon we visited at the time.
We miss the lifestyle occasionally, but are glad, overall, that we had the experience and decided not to continue it.
Eventually, it dawned on us that very few of the places we visited were as pleasant as where we lived, so we decided to give up the life. After four years, the last of which was the year of our 50th wedding anniversary, we sold the rig - we'd only used it for one trip in the spring of that year. It did come in useful as supplementary living accommodation for people from overseas who came for the anniversary (two from England and three from Japan).
Frank Damp, DW - Eileen, pet - female Labrador (10 yrs old), location Anacortes, WA, retired RVers (since Dec 2014)
