Here is what they had to do to fix my BLK Water Flush leak.
Remove cover of Water heater to access behind and above the heater, the black tank flush valve(Nozzle) was broken off the base where threads start. they removed, and ground out the fitting to flush valve(it was epoxied in place) trim the flange on new tank flush valve to fit, install and sealed the tank flush valve.
In short, the spot where the flush hose goes into the black tank had broken, so they had to replace it.
Considering I only have about 5K miles on my coach, it must have been that way for a while, and I just never noticed, because I usually flush the black tank and add water to the fresh water tank, so I was thinking it was from the fresh water fill, But yesterday, I was just flushing the black water tank.