Well, it explains a few things.
I seem to have a leak where the filler neck attaches to my freshwater tank. I can't tell without taking things apart whether it's the hose connecting the two or the material of the tank itself, but there seems to be a lot of silicone around the connector on the tank for the flexible hose, and possibly explains why the PO didn't have a key for the freshwater fill (that part is just my cynical supposition, but then again I'm pretty cynical.)
It also explains the crud on the interior skin below the seat of the dinette. I guess it's really old mildew (there's never been a smell of it while I had the trailer.) I'm leaving the seat off the dinette for now, and I'll put a fan there to help dry the area before I put it back together. It's at least going to be high 90s so that'll help with a quick dry too.
Looks like no boondocking in my immediate future!
(Not that I had any planned, but it was fun to think I could y'know.)
Here's what the area looks like: