We had a 2003 Rockwood popup that had a C-channel bumper (which is probably what most of the popups have). I mounted a bike rack (2 bikes) but just the weight of the bikes wanted to twist the bumper out so I had to strap the rack to the spare tire which was mounted directly to the back wall. Not saying this was right, just admitting what I did. It worked for the 2 years we had it.
If it's a C-channel type bumper, I would not mount anything on it. If it's a 4x4 square tube, you may be able to. Not all 4x4 bumpers are created equal, however, and others will be along with more specific advice on that, I'm sure.
Good luck!