I would start at the roof. Check all the sealant up there. Especially where the rubber roof rolls over the side behind the drip rail. Seal voids on or near the roof can allow water to enter the top of the wall or roof and travel down the slop of the roof to the low end of the coach, then down the interior of the wall.
The bubbles you are seeing is likely from water behind the wall penetrating the wall board.
That all said, it "could" be a leaking window, but my sense tells me to look elsewhere first.
On edit: After rereading your post, I skimmed by too quickly where you said the "back wall". Yes, lots of holes in it (window, screws for ladder, clearance lights).
I know this sounds drastic, but since the wall board is shot, consider removing it and keep going until you can tell where the water is getting to the back wall from.
