We have an old Jason brand fiberglass topper on one of our '95s. Typical specimen of that era, sliding side windows in the shape of a parallelogram. Got it for free several years ago with a broken side window. Found a replacement window (complete assembly) and installed it myself, but for the life of me I can't remember how I sealed it to the body of the topper. It's still rain-fast, but the other side, the original window, is leaking rain water in around the outside of the window frame.

When I installed the replacement, I ***THINK*** I recall just simply running a bead of silicone caulk on the inside of the topper body, around the edge of the opening, and the flange/frame of the window assembly sealed against that bead of caulk. I don't recall buying any tape or gasket or the like. Is that how this kind of window is typically sealed? This is one of those windows with about 20 screws on the interior side, fastening the frame to the topper body. No pics at my fingertips; I'll post one later if it's still light enough when I get home.

 

I don't know how they come sealed when new, but I would silicone it like you described. It's very typical for old toppers to leak.

 

Thanks dog, turns out it did have some sort of tape. And I bet it leaked since day one. The tape was not in the right position right at the corner where it's been leaking. Tape is between the outer frame flange and the outside surface of the topper.

Here's the kind of window I'm talking about, BTW:



This is the other side.

So now I wonder where I can find that tape.