455ci GMC, industrial A/T, reinforced Toronado drivetrain.  No driveshaft.
Six wheel braking.  MPG depends of course but owners claim 10MPG overall.
If you get a chance, ride in one.  Very impressive.

A greener and rarer alternative is the Turbodiesel Vixen.  Google Vixen
Motor Home for details  (Vixen with TD gets 30MPG according to owners).
http://stannerair.hypermart.net/