No, it's not automatic every so many years. Lots of environmental variables affect the longevity of window seals. But if one has gone bad, it's likely others aren't far behind.
No, caulk is a short-term band-aid, not a fix, and it just makes more clean-up work when you do a proper reseal.
Yes, it's very possible small amounts of water could be getting in unawares and rotting the wood mounting strips, running behind the foam liner, and causing slow deterioration of other things, even the wood inside the fiberglass floor if it finds a crack, screw hole, etc.
My thought is this. I wouldn't put the trailer out of service in the middle of prime camping season to do all the windows unless there were major problems, which doesn't seem to be the case here. But given the age of the trailer and the fact that one window has developed a leak, I think it's time to make plans to redo them all. The advantage of doing it proactively is you can do it when you want, not when a leak demands action.
Schedule a time when you won't be using the trailer, not too hot, too cold, or too wet. Allow plenty of time because it's very common to have to replace the wood strips around the windows. As you're preparing, I'm sure you can pull up some good old threads with details and pictures if you do a "Site Search/Google"* on something like "reseal Trillium
You'll feel so good when it's all done!
*Only available on the website, not the app. Much better than the app's search function.