I was talking to the RV tech at my local dealer about this exact thing. He said maybe a couple bikes on a light cargo rack but generators and firewood were too much. More than once he had seen the weight hanging off the back tweak the frame just enough to cause issues with the rear slide. I never even would have thought about that. When I asked him about it is first question was if I have a rear slide.
The weakness of the actual bumper is an entirely other issue. It pretty much got me off the idea of hanging much of anything off the bumper.