My Chevy Express-based Airstream Avenue didn't come with built-in screens except for the CRlaurence rear side windows added by Airstream. The factory Chevy Express flip-out windows came with nothing.
I found a company that makes a product they call "Skreens". Not cheap and they attach with magnets that go up pretty easy once you reach camp. The two for the rear door windows are essential when boondocking and keep out blowing rain as well. At least ours have so far.
See my posts in this prior screen discussion thread from a few years ago.