An RV isn't a car. There are those who use them for business. One of my favorites is the old FMC. Built in limited quantity (and high price) in the 1970s. Low ground clearance, independent suspension, low roofline and rear engine. A early Duramax conversion can be worth almost 50% in fuel reduction (gallons, not fuel price). On a rig that was paid for long ago it isn't such an expense given that house is home is car as well. With a concurrent change to disc brakes one is looking at a 30' moho getting 15+ mpg that can travel high speed in safety (65-70 mph).
TheDieselPage has
BigRabbitMan's conversion,
here.
The choice of coach is more important than the drivetrain, IMO.