The rubber gasketing on the outside between the frame and the glass is not what seals these windows. The mastic material that holds the glass in is also the seal; the rubber is mainly trim and edge protection and a little support. They are built somewhat like auto windshields in that regard.
If you (or an RV shop) determine the make of the window, finding the proper rubber should not be a huge difficulty. It might be possible to figure the correct type based on a piece of the original and its cross section.
If you can find a make on the other windows in the RV, there's a good chance that they would be the same throughout.
