For future readers.... Transmitted flexing showing up as missing rivets above the main entry door can flag a whole list of possible causes needing checking...
On old trailers a peek at outriggers and their through-bolt fasteners would be warranted when pop-rivets go missing and/or oxide black rings appear around them.
With the banana wrap system it too easy for previous owners to replace the cover sheet and leave chaos nicely covered up from accidents or blowouts.
My '73 trailer had sixish missing pop-rivets and same number with black rings, and we found busted/torn/un-welded outriggers plus C-channel bolts loose from rotten plywood and rusted/missing bolts. Just saying 'cuss it needs to be said