Since you didn't put in your make and model rv I am not sure if this will help. On mine you can reach a few inches up the pipe and see the knife valve. I can easily spray a strong stream of water into the slot the valve blade goes into. If you have some crud in the rubber seal area and it can't close this could (might?) clean it out. Maybe even a gentle scrape with a wire if you can reach it. Beyond that , replace it. $15 and watch a few youtubes to figure out how to do it (after you empty the tank, LOL).
2017 SLX 195RB
2014 Jeep Grand Cherokee Overland, gas 3.6 V6
Andersen WDH hitch
Dometic Milenco Aero mirrors