DO you want the bad news, the really bad news, or the probably going to make your eyes roll news first?
1 - you have a leak, maybe more than one. Water is getting in along the edge of the rear cap. Most common leak is from above, but it could be from the side. This has to be tracked down and sealed.
2- once that rear cap starts to delaminate, it will not stop. it will eventually have to be replaced (our trailer is at dealer for a new rear cap right now).
3- technically, there is no warranty whatsoever on your unit. The stated policy of Forest River, and it has been quoted many times, is that they do not honor warranty to subsequent purchaser. The original owner would have had a potential claim (most sidewalls and end cap have a multi-year warranty)
Sorry - none of that is good news. You may be able to put claim through on your insurance. Each one is different on water damage.
2013 Coachmen Freedom Express 320BHDS pulled by a 2005 F250 King Ranch CC
A rainy day camping is better than a sunny day at work.