More than likely that's water damage. You're going to need to open that area up and see what's in there. I ended up taking out the entire rear section of paneling in my T1661, removed all the insulation and rotted lumber (including a good portion of the frame work) and reworked everything in there. There was a musky smell from the wet rottted lumber (most likely mold) and once I reworked everything that smell went away.

Also, by removing the paneling and such, I was able to see the actual hole that was letting in the rain; it was almost dime size in diameter, and I fixed it by using elastic repair patch tape and then put three coats of rubber roof sealant over that. No leaks now.

I still have a portion I have to repair in the front end of the camper as well, because I have more sagging paneling there...another leak! Good luck!