Most RV roofs are strong due to a process used to pressure seal the inner and outer panels along with all of the insulation together using glue and a vacuum process. This creates a strong platform which distributes weight across the roof. You would have to drill through the roof and put a plate on top of the roof to spread the weight across the roof. Anytime you penetrate the roof you create the possibility of a leak. There aren't the typical rafters to screw into like in a home.
Your best bet would be to use a stand like this
http://www.yogaswings.com/
Or have something built to fit in the area of your RV.
