I saw an Ultravan for the first time last weekend.
It seems to be fairly aero. It is made out of aluminum and supposedly fairly light.
Some google image pics:
After cleaning up the body, get rid of the awning and A/C, it might make a reasonably aero shape. Maybe drop a VW TDI in there.
This could be somewhat close to a Dymaxion type vehicle. Except for rear wheel steering, which is better left alone.