I would contact the trailer manufacturer, give them the VIN number at tell them which side, a brief discription of the window and give them your window measurments. They should have some kind of record of the materials used to build your trailer.....I would guess.

 Do you need the whole window, the frame as well as the lens/glass? OR, just the glass. If just the glass, then run the trailer by your local auto glass shop and have them replace it.

The window is measured to the inside of the trailer frame. You will need the height and width. Do not include the width of the rubber flange if it has one. If you are still unsure, make a paper template and show that to whomever you will use as a supplier.

BOL  Gard