I had a leak in my
Trillium 1300 that showed up in a similar spot - noticed a damp cushion in the back corner (standing water once the cushions were removed). It's been two years now and I'm still not 100% convinced I've found and fixed the source...
I took out and resealed the back and two side windows as well as the back window pane itself (the back window isn't original and I believe some of the caulking was missing), I had the lights off and sealed around the wires, I sealed all the rivet heads around the belly band, sealed (epoxy putty) a myriad of rock pock marks around the trailer (I was on a roll) and even a couple of divots in the FG on the roof.
Last year I left the trailer parked over the winter with it non-level, tilting down towards the hitch and when I inspected after the first severe winter rain storm - no wet spots. Last weekend I leveled the trailer to see if that made any difference...I'm still trying to convince myself I fixed the leak somewhere in my trek. (It's rained pretty good since then and I'm going to run out and check it once I've sent this...thanks for the reminder)
I was convinced it was the window but the leak persisted after I'd replaced the window's perimeter seal, that led me to check the window glass installation itself, that led to... short story is, the source of the leak wasn't obvious in my case. Hopefully yours will be different, but do be prepared to look and consider a number of things - it seems water can travel a distance under the wall covering (mine's the ensolite stuff).
Good Luck in your hunt.