Those things are prone to fail. I actually replaced mine with a Watts brass unit that can be rebuilt, and added an inline check valve. It is a vacuum breaker/checkvalve. Basically, it is there so when you flush your tank, and shut off the water source, it does not let black water siphon back through the flush line. I bought mine off Amazon, and they have them basically just like yours.
Also, too much water pressure will make it leak. try the teflon tape trick and use less pressure.see how that works.
It was a rainy few days at Algonquin Prov park...tarps kept us dry.
Wayne --Belle River (Windsor), Ontario
2013 Sunset Trail Reserve 25RB..
2015 White Chev Silverado 2500HD 4x4 (6.0 l gasser)
2003 Mountain Star 890SBRX Truck Camper
All of that makes sense.
I probably had the pressure too high to begin with, but after it started leaking and I was checking to see where it was coming from I had the pressure low and it still leaked water.
I'm assuming the high pressure to begin with damaged the unit, and then even with low pressure the valve failed. Does this seem plausible?