Greetings Dolphina,
I believe the rear window is the same as my Seabreeze. It makes sense that it would be plastic for safety reasons,e.g. being rear-ended would eject the unlucky person using the restroom at the wrong moment etc. I have been jostled around a few times using the restroom in transit. Usually I slam into the closet door mirror. Hasn't broke yet.
As for the leak, I would bet its the window seal. Remove screws on the inside frame and gently pry out window frame from the outside. Remove all the old butyl tape from frame and around window opening. Install new butyl tape around perimeter of frame. Carefully install window, centering it on opening. Reinstall interior ring w/screws. Done deal, As an added precaution I run a bead of non silicone caulking around exterior frame, When you buy the butyl tape at an RV supply house. they should hook you up with the proper caulking. I found out the hard way, that siliconized caulking is a no no on RV exteriors. It does not work well,is super hard to remove, and makes your RV look like oooo when it starts collecting dirt and dust like a magnet.
Happy trails,
seabreezer