How much mpg does a typical motorhome do?

HEDONIST

OK, I know it is like asking how long is a piece of string, but what sort of mpg would one realistically expect to get from a regular motorhome. By regular I mean exluding the huge Winnebago sized outfits.  Something based on a Transit sized chassis or similar.

Just a guideline would be great. Only really interested in diesel engined vehicles.