Vulcan,
I noticed under my Jayco, that the marine ply has all sorts of funny work done on it. I have screws that have come through and a couple of screw holes that expose the interior. Makes me wonder what would happen if I was to drive through water and splash it up and under the trailer.
Other than that I would have no idea how it would get in there.
Guessing on what others have said about the quality of workmanship at Jayco in other posts - it wouldn't surprise me if/how water gets in (but I still love my Jayco).
Cheers, Paul