The ones I've seen are identical to those in pop-up tent trailers. Admittedly, I haven't looked at either in many years, but they used to be roll up canvas with the built in screens.

They usually do a pretty good job of keeping out the rain. I don't think you'd really want solid panels, as you'd have poor ventilation, and it would get really hot inside when parked in the sun.

Good point about the mast. I know of at least two people that have broken their rear RV windows with masts...

Mike