How are you misting the trailer to check for the leak? Starting at the bottom and working your way up? That would be the best way to check. Maybe you have a leaking rivet, or series of rivets. Or, maybe the leak is coming under the rub rail that wraps around the bottom of the shell. I would focus my attention there, misting slowly along the rub rail from front to back. If no leak develops, then move up the side of the trailer, again going slowly from front to back.
Good Luck! Hidden leaks can be sooooooooo frustrating!