This is the response I received from Reace a few months back after I inquired about replacing the rear window in my 2015 17B with an opening window:
"Installing a sliding window in the rear of the trailer will only lead to leaks in a short period as the window drain track will quickly plug up from road wash. The frameless windows are only available on the 2017 models as we had to make changes to the trailer design and fiberglass layup to accommodate them.
The only option would be to install an awning style window in the same style as your windows. There are a few different varieties, however none of these would be available in a thermal window. We would not be able to provide one of these windows, you will need to contact Hehr directly."