If I didn't have silicone around the windows I'd never use it on there. It's too hard to reseal when it starts leaking.
If I already had silicone around my windows I'm not sure I'd start digging it out of it was in good shape. I think it's something I'd start doing one window at a time and would try to replace it as quickly as I could tolerate that work. Silicone is a PITA to remove completely. Just know that eventually the silicone will fail and you'll need to cleanup the mess then (hopefully with no water damage).
My windows were sealed with silicone when I picked it up and the windows were leaking. There were more issues than just the silicone failing, but I pulled the windows which made cleanup of the silicone easier. I wanted to reinstall with butyl tape instead of the deteriorating foam that the trailer came with.
For the record, I use Proflex for all vertical seams on the outside of our trailer. Try it once and you'll see why this is a superior product over silicone for this application.