It may not be fully closed after the previous dump procedure. I have a fresh tank valve that doesn't quite close all the way unless I "coax" it a little with a rubber mallet. I usually notice a drip at the exit port underneath the Navion on my driveway. It's fresh water, so I don't worry about it too much, unlike your situation.
I'm not sure any of the blade valves that were used in the various fluid systems builds were top end stuff.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)