I had the same thing in our Scamp
. I replaced the rear window, removed and re-sealed the side windows
, replaced the rivets, re-sealed the top vent....all to find out it was the belly band.
I removed the wall covering and found that the water was entering at the belly band on the side of the trailer and running inside the fiberglass until it got to the back piece of wood, then poured out through that down to the floor.
I ended up using an assortment of dental picks and removing all the RTV along the top side of the belly band (making sure I got it all out, even under the lip of the belly band). I also cleaned the fiberglass real good with acetone. Then I redid the RTV using ProFlex RV (it sticks very well to the fiberglass and doesn't craze it over time). It was a long and tedious process, but it's been dry ever since!
When I replace the rear wood, I sealed the wood with several coats of fiberglass resin before fiberglassing it back into the trailer. At least that way it would be less likely to rot out again if the belly band ever leaks
again. Also, be sure to seal the two plastic bases that go under the screws that hold the wood in position while you fiberglass it.
Only seal the top lip of the belly band, not the bottom.