That might be problematic since Casita
reshaped their trailer molds back in 2003, and the rear window they now use, and have used since then, is taller and narrower than the older window units. You might find yourself looking to either have a special tempered window pane custom made ($$$) or else, look through the RV window catalogs for on that will fit. Most likely, there won't be a direct match, but several manufacturers will custom make a new window to your specs, including corner radii, tint packages, frame color, etc.
I've bought a small custom window, (which I installed in the front of my Casita
so I can see out the front without having to go outside,) from this outfit. They'll custom make just about anything you could want, and they're close by.