I'm not familiar with U-Hauls but I just eliminated the same problem on a 1988 Lil Bigfoot
I removed, dismantled reapired and replaced every window in the trailer.
The sealant between the glass and the aluminum window frame had dried and was leaking.
The exterior rubber moulding is not a water seal it's a trim.
Remove the glass from the window frame,clean the old sealant from the glass and window frame.
Apply a small bead of black silicone to the frame and replace the glass in the frame it should eliminate the leakage.
It's a lot of work, but well worth it.