In 1990, I believe the walls had wood framing in the walls. This is area I would be concern about. I'm not sure about portholes( that would be a boat ), the bottom rails of the upper section may have dry rot below the windows causing it to roll under (the wall will twist inward ). If the wall is wavey above the window, its a roof leak. This a costly repair because you have to rebuild the walls in the whole upper half. The most common cause would a water leak from the roof or window. It's a tough call because sometimes you don't know how bad the water damage is until you start opening the walls for the repair. This sounds like what has happened to my 97 towlite, ( dry rot of the bottom rail )I don't have an area at my house to rebuild it. As of right now I'm not sure what I'm going to do. It's still in campable condition,so I plan to enjoy it.
2006 22ft towlite (replaces 97 21TL)
2006 Toyota tacoma pre-runner 4.0 L V-6 quad cab
w/ 98 chevy Powervision towing mirrors