[jcurtis934]

If the small black plastic device has a pex line coming into it and one going out of it...that is the black tank rinse anti-siphon valve. Mine always spurted when I first turned on the rinse function. If you are saying that turning on the sink faucet is what you believe to be causing the moisture, then it could also be water coming out of the top of the faucet...under the handle, which is due to the ball that controls water entering in the faucet. In this case, water leaks out and flows down the fixture, under the fixture and follows the water lines as it drips. This is what occurred on ours and it was hard to tell that the water was coming from there...unless you looked close. If this is your cause, then shut off water, remove the handle (assume single faucet as ours), tighten the top gland nut that holds the ball control. The metal stem that the handle fits onto should be harder to move by hand. Put handle on, turn water on, leak should be gone. IF it is the black tank rinse anti-siphon valve, I removed mine and used brass fittings to connect the lines back together. Then on the outside of the rv, I have a anti-siphon valve there. John