WOW! This has been on my list to figure out why mine was doing this too.. Since I didn't have the words to describe it, I have just postponed jumping into it. I try to avoid all dealers at all costs
. Frame flex.. Thanks for posting this, OP.
On that note, it seems like the (common) solution is to replace the screws. Not having my 5er near, I seem to remember there was a "panel" on the covering underside of the RV around the hitch. Should that be removed, and then there are screws (not visible otherwise) that should be replaced with longer ones? OR, are the screws visible from the outside the ones that should be replaced? I seem to remember they were not very long to begin with.
Thanks!