My electric gray tank valve was not working while the black tank valve and the pump did operate.
The light will not light since the switch for is triggered by the valve not being closed.
Doug at Phoenix USA in Elkhart replaced the valve.
In spite of how I understood it, it turns that there are no manual drain valves as such.
Doug said you have to unscrew the motor on the valve, unscrew the part that actuates it - and toss it out - and you can slide the gate valve manually.
Good luck with that,