I was one of the early posters about this. I was luck enough to catch it while still in warranty and Thor paid for camping world to replace the entire back wall. and mine wasn't really all that bad in the big scheme of things. Caught it fairly early.
The color for the panel was discontinued so they did it in white and painted to match. That painted panel held up far better than whatever the gelcoat they use, and is the best looking part of the rig now, by far!
I'm still kicking myself for not asking for surface mounted tail lights instead of going back with the same.... EVERY single time I check on the rig I look at those lights for condensation..... just did today!
Later, I found some rot in one of the wing walls, behind the passenger side cab door. It was more extensive...but of course out of warranty. I was able to remove the rot, treat the remaining wood with rot doctor epoxy
and do a scab in repair without damaging the fiberglass finish face.
I'd imagine a similar repair could be made to the back wall by removing the wall paper inside and getting at it that way.
I always thought that if it ever came to it, and the outside needed to be cut away then I would fabricate some plates of something and just cover the rear wall or a potion of it and mount surface lights to it.... maybe a diamond plate sheet, or a sheet of some nice decorative plastic. It wouldn't look stock but could be done to look nice.