I've just removed and reconditioned all the windows in my Lil Bigfoot
I have replaced the front and rear plexi glass and replaced the fixed pane in the large side window.
The rubber mouldings that hold the window in place do not seal out the water.
There is a sealant used between the glass and the frame.
Today black silicone is used.
The rear plexiglass window leaked and it was obvious when I removed it the sealant had dried up.
Gently pull the moulding away, remove the glass clean it and the frame.
Run a bead of black silicone around the frame and reinstall the glass.
It's a much easier job if the windows are removed.
Make sure whe weep holes in the bottom of the frame are clean.