[link|http://www.fleetwoodrv-info.com/bdrd/bdrd_specs.asp?model=BDRD39Z|Fleetwood Bounder Diesel] specs.
[link|http://www.uniquemotorsports.com/calculators.html|Horsepower required to overcome air resistance] calculator.
Aero HP = 0.000007 x Frontal Area x Cd x speed^3
= 0.000007 x 102 sq. ft x 0.5? x 126 x 126 x 126
= 714 HP
I doubt that they're getting that kind of power out of a 5.9 liter diesel. Then you still need to overcome the friction of the tires, etc., for the 21,000 pound beast.
Heck, even dropping it from a C-17 at 30,000 feet probably wouldn't make it go that fast! :-)
Cheers,
Scott.