Back in my fifth wheel days (my first rv), there was no way to look through the rear window directly. I had to use all the mirrors, only. I could see the pin box in the main mirror, although not the hitch. The biggest help was to make a "dot" using black tape in the dead center top of the rear window. Using my mirror, if the dot was lined up with the pin, and then if I had exactly equal amounts of trailer sides (with no trailer rear bumper showing) in each of the side mirrors, I was good to go. If need be, you could combine this trick with a white tape centerline on the pin box, if you can't see the pin...