What you are looking for is a rubber window molding with a "lock strip," called Self-Sealing Weatherstrip.
Basically how it works is that the molding has a slit in the middle of one face. You ignore that, install the window and molding like any other, and then after that come back and "force" a triangular piece of rubber into the slit. This forces the gasket apart and makes it seal well. This same style is used on things like school bus and heavy equipment windows.
Here is a company that makes these moldings. From them you can get one that fits your trailer and window just right. You measure how thick your trailer's fiberglass shell is and also how thick your window plastic is (or will be) and then order to suit.
In addition to the traditional separate lockstrip, there is a style with a "built in" lockstrip. It still works the same way, but it's kind of "hinged" and just flops down and gets wedged in.
One look you might like: In the separate lockstrip style, you can get a shiny silver lockstrip, for a bit of an accent. Works the same way as the black rubber one; just looks different. There is a "lockstrip tool" (they make one version) that helps to get the strip in.
Here is a link; look at the second row down, the second, third, and fourth items over:
CRL Auto Glass Weatherstrips and Moldings
Here are the tools:
CRL Weatherstrip Installation Tools
I don't know if you will be able to order from them or if you will go through your shop; but either way the golden ring is a few part numbers
Your body shop folks will probably be familiar with ordering from people such as CRL.
PS: This is the same basic stuff that Scamp
sells (they may get it from CR for all I know); but here you have more options for suiting the exact thickness of your trailer and window, plus the length. Also you could get the "chrome" lockstrip if you wanted to.