Wow, that last thread is somewhat dismal. (don't cash in the unit yet
Actually, you symptom is typical of that coach. ( I had the same coach with the exact same problem. I had to replace the rear bedroom floor.
When shopping for older airstreams, this is the first place I look for water damge. The entrance door is almost a given seeing that the door is not the best seal after a few years of use.
The rear window can leak through the sill and even more common, where the exterior skin meets the bumper will seperate and water will seep under the shell and wick into the wooden floor.
if the damage is not too far gone, you can dry the area well and treat with a wood treatment agent that will harden the deteriorating wood. The make certain that you seal the bumper and the exterior skin.
Obviously this is all for not if you have a lot of vibration from unbalanced road gear and even worse, the dreaded "rear end seperation"
You can read plenty about that on this forum starting witha thread called "Droopy rear end" or "rear end sag"
Dont get too scared yet, it all comes with vintage ownership.