Nugget,
Do you have a son or friend that can do a bit of work on your RV?
Problem you're having is not uncommon. The manual valve has an 8 ft long cable, that you can't lube, and is probably twice as long as it needs to be. Thus it hangs up an eventually something bad happens and you can't open or close the valve.
I have two suggestions:
1) Get a Valterra "Twist On" valve that goes on the end of the drain piping...where you connect your flex drain valve. With the tanks drained or valves closed, remove the cap, twist the new valve on, and move the cap to the discharge of the new "Twist On" valve. Then just leave the black tank valve open and use the "Twist On" valve as your black tank valve.
Here's a link to the "Twist On" valve:
Twist On Valve Archives - Valterra.com | Valterra.com
For $26.52 hard to beat.
The other option is to have someone open the belly and replace the manual valve with the Valterra electric valve. Then just flip a switch and you're draining the tank, flip it closed and the valve closes and seals very well. But at $246.18 not cheap...but work just excellent!! I have them on black and grey water tanks!
Here's a link to the electric valves:
EZ-Valve Archives - Valterra.com | Valterra.com
Just a couple of thoughts,
Pirate
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