mowermech, after some thought on the matter I agree, because the rear bumper on my 5th wheel is not really a bumper, rather it is a tube for storing a sewer hose. It is very thin and I would not feel safe attaching a hitch to it. I was planning on attaching a cargo carrier to it but changed my mind.
That said, I have seen people who double hitch a 5th wheel, towing a small boat behind their trailer, but I’m sure it was not done with a bumper mounted hitch, rather they used a frame mounted hitch; which in my opinion is also dangerous.
I do have my 5th wheel spare tire mounted on the rear bumper-like storage tube, and have had no problems with it as yet. It came from the dealer that way with the spare tire mount attached to the rear bumper tube.