Since it WAS draining and then STOPPED it sounds like the 'cut-out' disc was left in the tank and has now been pulled down to the tank outlet blocking the flow
'Cut-Out' disc....when toilet line to tank, vent line to tank, outlet line from tank are installed they cut out the opening. Discs are not suppose to be allowed to fall inside tank....but floor jockey gets paid by numbers and if disc falls inside Oh Well!
If it is the disc you will continue to have ongoing draining issues
Have to disassemble dump valve/drain pipe etc to access the tank outlet and remove the disc
Other then Disc blocking I can't think of another reason for perfectl y draining tank to just STOP
IF you had closed the dump valve and then when opening it tank didn't drain then I would say dump valve is not opening
But you stated it stopped draining and THEN you closed the valve
**Should never walk away when using flush connection
I took my Medication today. HAVE YOU?
Dodge 3500 w/Tractor Motor & NUWA 5vr
US NAVY---USS Decatur DDG-31