Yep, it happens
It depends on the year your unit was made -- but some of the units from the early 2000s were kind of skimpy on the caulking around the window. If you search RV.net, you can find many discussions of what to do.
On my old 2003, I carefully unscrewed the window frame, caulked around the perimeter, and reinstalled the window -- you may need two people for the removal and the reinstall. Problem solved.
My newer trailer, so far, is leak free. Knock on wood (or fiberglass, actually).