Your sink is on the side of the trailer, right? (Mine's on the rear in a 17T).
I doubt the problem is due to plumbing if the damage is on the part that is attached to the upper half of the trailer. There's no plumbing up there. I suspect you have a leak in the upper half that has let rain water in, possibly around the window, or along the edge of the roof - there's a seam up there.
If it's like mine, that piece is a strip of vinyl (wood grain) covered wood that has bee attached with a gummy contact glue. It can be lifted off, starting at one end or the other and you can inspect for further damage.
There's a couple of other threads in this forum where people have replaced the underlying side of the top that has been water damaged. It looks like a BIG job to me. If the damage is localized, I'd probably just try to stop the source of the leak and trust that it wouldn't spread.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew