There was an RV that Motor Week profiled a few weeks ago that was a Dodge variant that got I believe something like 16 MPG. Granted it was one of those smaller 2-3 person sleepers, but I was pretty impressed by that.
My uncle had a 46' bus style and I remember my father telling me on trips with him that they were happy that they got good gas milage on one of the legs of the trip because they got 5 MPG. Granted this was like 20 years ago but even back then I was perplexed at how this was "good"....
