Answer:
The average mpg of a vw camper van is 18 - 20 mpg
Older campers had smaller engines, where slower, but got better gas mileage.
A Vanagon camper van (1984-1991) got around 18-20mpg. The 1980-1983 Vanagons did worse, but the 1968-1979 baywindow buses did better. the early baywindows (1968-1971) and splitties (1967 and earlier) could get 24 mpg on the highway.