This may not be your problem, but our UHaul
had water on the floor with every rain until I found the culprit: Our door didn't have a tight seal at the top when closed. The rain would run down over the lip of the wall about the door, then run down the inside of the door onto the floor. We added a stick-on gutter over the door (EZ-Gutter?) and weatherstripping in the door frame and on the door. No more leaks!
We also had a window that leaked because the original outside seal was gone. We used black butyl caulk and it has been fine. You may want to spray your trailer with a hose to find the visible leaks
. This is more force than a rainstorm will give you and someone can sit inside as you spray each window for a few minutes. Don't forget to spray the roof vent, too.
Hope you find the problem! A dry trailer is nice!