[JRTJH]

I believe you have a 2015 Cougar ??? If so, and if you didn't winterize or remove that valve last winter, chances are it froze and is broken. It is the black tank anti-siphon valve, designed to prevent backflow of black tank contents/gasses into the city water supply if you left the black tank flush connector attached to your hose.

I'd think, if it works when you flush your black tank, it isn't connected backwards. That leaves loose connections, usually they are only hand tightened, so you can reach under the sink and tighten them sufficiently. If that doesn't work, then it's probably freeze damage and the valve needs to be replaced. There isn't any repair that I know of. You can find an exact replacement at any RV parts store, Amazon, EBay or most of the internet RV parts sites. You can also, as Danny said, go to Lowe's/Home Depot and get a replacement in the plumbing supply or irrigation section.

I wouldn't delay the repair if you intend to use your black tank flush system. If it's cracked and leaking, it's only a matter of time before the crack becomes a split and you "flush your entire bathroom"... Even though it's clean water, that winds up being a mess to clean up.