Cause of puckering and repair.

Many thanks to all of you for responding so quickly with good information.
I was looking to buy the trailer but I saw the delamination and slowed down a little. :E I will search "delamination repairs" for more info
It looked like water damage to me also, and the info about the heat makes sense to me too.
There didn't seem to be any evidence of a water leak in the interior.
Drat the luck! It was a nice rig for my purposes.

Thanks again all!!

Not sure if your Cougar has a vacuum bonded rear wall. Sometimes the filon is only held on by perimeter fastening and the big window. Our Heartland Northtrail was that way. Actually had a couple small puckers after only 2 yrs.

If that's the case it wouldn't be too tough of a repair.

I see that you are currently located in Las Vegas.
Before we get too far down the water intrusion road, I will offer up the opinion that HEAT could have caused the delamination. I have been located in Furnace Creek for the past 2 1/2 years and several of the RVs with smooth side laminated side walls located here full time have this issue. It usually happens on the south or west facing sides where the sun beats the hardest. That heat will loosen the glues and the "plastic" surface separates from the inner material. It will then buckle and pooch. Come on down and look around in the employee section of the resort and you will see what I mean.

I would start by saying that you appear to have a severe case of delamination caused you one or more water leaks at the rear roof/end-wall seam area. Water gets in behind the thin filon layer and breaks down the glue and causes the filon to separate
from the thin "cardboard" backing material.
You will likely need to remove and replace the entire rear wall. First you need to identify and repair the leak source.
Do a search on this forum for "delamination repair" and am certain you will get some ideas.