When you say "trim," do you mean the rubber strip that fits into a groove?
If you DO mean the rubber gasket cover strip...
Our strips were shrunken and deteriorated, and a Previous Owner had painted over the belly band portion with brown enamel paint
so that it was brittle and crackling and cleaning it looked like a nightmare.
We couldn't readily find quite what we needed and searched around quite a bit. Also we were fixing up the whole trailer and cost was an issue.
Finally we just cleaned & painted INSIDE the groove with black enamel, over the newly-replaced and well-sealed screw heads.
Photo shows rear window with paint
inside the channel, not rubber. If you're restoring to vintage, then this isn't right. But for merely camping, you can do it however you like, and you don't even have to have a reason.
The trim strip is ONLY decorative. It doesn't even protect the screws, as it holds dirt and moisture up against them constantly. The paint
looks about as nice. Good enough, anyway. At least we think so. As we say here, YMMV: Your Mileage May Vary. Or, To Each His Own. (Though we never write, TEHO...though we could.)
Good luck with your fix-ups and happy camping!