Water can travel a ways. Is this from when the unit was stored or are you full timing so it's level all the time?
A couple ideas. If the slides are out and it's leaking look at the wiper slides to see if there is anything keeping them from making good contact with the slide walls. Water can track in that way.
Check the windows. On our rig, none that is ZERO windows had been sealed with caulk around the edges. There was a bead on the top flat edge but that was all. We had a window in the slide that leaked like a water wall when it rained. The factory repair was to put a bead around the top and sides, then the window started leaking shortly after. I pulled the window and found that the seal putty they used was installed in the side edges not on the flange that mates with the wall. Wide open water access to the wall and inside of the trailer.
Look at the roof seam on the cap where the roof mates with the fiberglass. There may be cracks in the caulking or the seam may be lifting up. The factory assembly workers had a habit of using small screws and breaking them during assembly. They then stick the broken head of the screw into a dab of caulk and moving on. I have lost count of the number of broken screws on my rig. I now check the roof and caulk lines at least every 2 or 3 months looking for cracks.
Check the sides of the rig for broken screws on the trim strip that goes on the mating surface of the cap and sidewalls. If there is a crack in the caulking there or the strip is lifting from broken screws water can get in there as well.
Those are the most likely sources of water at the rear of the rig.
Mike Nancy and the fuzzies
Fulltiming since June 2004
Volvo 660 MH tow vehicle
2005 MS 38RL
2007 Saturn Ion "toad"
2010 Gold Wing "piggyback"