Just a thought, but something worth a check, is the outside kitchen sink, put a bowl under it to see if it drips a little water, with the stopper in it, PLUS also when you allow the water to drain.. I mention this as last summer our outside kitchen was dripping only when allowing the water to drain, and not when the stopper was in place. PLUS when I researched and repaired my situation the HepvO waterless P-trap was also installed upside-down, I realize you have a different trailer but worth the time to check, My leak was even a little tricky to detect as until I reached right to the back of the cupboard no moisture was evident, then it took a while to figure out where it was coming from, Once found the fix was fairly simple, Turned the waterless p-trap the right side up, and since the hose used is so stiff I gave that the proper conture and retightened everything as it should be, problem was solved, no expense just needed a little fine tuning. and I may have added some Vaseline to get a tighter seal with the plastic threaded donuts that hold things together.
2012 ST25RB SunSet Trail - RESERVE
Silverado LT Z71