I've always thought a 40 foot step van would be pretty spiffy.I want to get a 28' Uhaul truck and convert it to an RV. There are quite a few write-ups floating around the internet about them. Unfortunately the diesels are becoming scarce, and the gassers measure fuel consumption in gallons per mile (exaggeration).
Especially if you could give it 4 wheel drive.
sliding glass doors at the rear with a fold down deck that locks everything in place when folded up.