Pressure valve? leak on Cougar

[Howie]

Can someone help me on what this valve is? It is located underneath the bathroom sink that has 2 white water lines running to it. When I attached the hose outside to do a power flush of the black tank I noticed water running down the white lines from the bathroom down into the convenience center in the belly storage. The lines go straight up to the underside of the sink and the black valve sits with the black protrusion on the top of the valve. I found that this valve was leaking and I don't know if it is the attachment threads or the valve itself yet. It does not leak when I am attached to city water, only when flushing the black tank. I will investigate tomorrow when I can get back to it. I attempted to attach a picture with no success.