I won't comment on the roof installation from a mechanical stand point. I will say if you install on the roof you will see a significant reduction in the usefulness of the antenna. Satellite signals are line of sight and ANYTHING between you and the satellite will cut down or completely shut off the signal.
Think about where you camp before you buy.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles