We had a trailer that we'd owned for 14 years, needless to say we enjoyed it a lot. We live in an area that's known for rough weather, and at one time suspected a leak in a window or two; so I can help out a bit here.
The windows are typically two-piece, with an outer section that does the sealing to the outside of the unit - it has the glass and screens - and an inner ring, which holds it all in place. It's just a few minutes' work with a screw gun to take the inner ring off and then to push the window into the arms of a helper.
To re-seal, just apply
butyl rubber tape, like this product on Anazon, then put the window back together. You'll want to measure your flanges beforehand to make sure you have the right size tape - size the tape to the flange, 25% smaller, to allow for the butyl to squeeze out as the window comes back together. The butyl will squeeze out of the flange, making a perfect seal. Best thing about butyl tape is that you can cut-and-piece it together when resealing things like outside power outlets. (Yes, don't forget that kind of stuff.)
You'll want a plastic putty knife and plenty of paper towels for cleanup, but the reseal will last a dozen years or more.
Hope that helps.
