The Ford Transit is RWD also the Nissan work van, the Promaster would be nice for a build your own, it has Front wheel drive however the rear axle hangs down quite low for ground clearance, I would worry about running aground on bad roads...
If you fix a Promaster up yourself, you can later rip it all out and still have a useable van for something besides an RV.
To get on the road without all the usual RV frills in the Roadtreks etc would be quite simple. A few frames for beds or one bed, portable 2 burner stove, commercial sink with a hand or foot pump, some cupboards, a few insulated windows, Fantastic fan,,
cassette toilet, furnace, insulate properly ( not like most of the commercial rv's..)
Put on a few solar panels and lots of good quality battery capacity.
Good Examples here..