I installed my Renogy flexible 100 watt on the roof of my Alaskan using "extreme titanium" velcro. It works fine for wind, I don't drive down the road with the panel mounted and I don't plan to do that experiment in the future. I wanted a setup where I could also park in the shade and put the panel nearby in the sun.
One day, astounded by my own brilliance, I just leaned the panel against the front windshield of the truck so that I was getting optimum angle of the panel AND ventilation behind the panel. How smart is that? I came back at the end of the day and the panel had nasty creases along lines between the mounting rivets. The panel still works but it sure looks ugly now. It looks like I tried to fold it up and and carry it home under my arm.
Apparently these things need to be continuously supported by a solid surface.