If the engine is EFI,I don't see an issue. If it has a caburator, run as fast as you can as far away as you can get. Vehicles built with a carb in the final couple of years before fuel injection became standard have so much emission control **** that they are almost impossible to maintain and get really poor gas mileage.
Our first rig was an '88 Class C, a 26-footer with a Ford 460 V8 with carbs. No matter who "tuned" it, the best gas mileage we got was 6 mpg at sea level. In the mountains, it rarely bettered 4.5 mpg.
At the time, gas was around $1.80/gal, so we were able to sell the piece of junk quite quickly. We replaced it with a 32' Class A that had an EFI Ford 6.8l V-8. That got a consistent 8 mpg that didn't change with altitude.
Since we have mountain passes at 5,200', 4,400' and 2,000' within 120 miles of home and we preferred to travel east of the Cascades, that was quite important.