All my "windows that open" have a screen, even the "hinged at the top EXIT windows".... If it opens, there's a screen...
So, I'd suspect that your trailer originally had a screen, but what happened to it and finding a replacement may not be an easy task.
You can start on the Trekwood.com website, search by your trailer year/brand. There's a section for "windows" and in that section, all (or most) of the windows used on that year and brand are listed. Toward the back of the section, about 7 or 8 pages in, is a section on screens. They're listed by size and location (top, vertical, horizontal screen, etc)... If you can find the size, use that part number to start searching on Google for a cheaper price....
Or, possibly you can locate a "local glass and window company" that makes custom screens for houses. They should be able to build any shape, any radius and any size screen and probably cheaper than ordering one from an online RV parts store....
Here's the link to Trekwood's window listing for your trailer:
https://www.trekwood.com/parts-searc...arch-keywords=