Do keep in mind that a variety of sources will show up there. That is the c-channel junction between the curved rear and the straight side. Water that enters the c-channel will run inside the channel to that opening. But since it is occurring stationary and presumably level, look to those awning mounts, the drip rail, the seam between the rear clam shell and the roof/side wall, the FF or A/C (whichever you have). Touch all the rivets in the area with Captain Tolley's.
These are "hunt and find" exercises. Assume frustration going in and be patient.
"If the women don't find you handsome, they should at least find you handy." - Red Green