Replacing black tank knife valve Holiday Rambler Vacationer

The black knife valve on my 2004 Holiday Rambler Vacationer 36WDD does not seal tightly, and juice seeps down in the drain. It is a pull/push cable actuated unit and is way back in the wet bay out of sight. Does anybody know how it can be accessed for service or replacement? I have tried numerous methods to clean it from above through the toilet without success. I currently have a short section of old garden hose connected to the drain cap, that is equipped with a 3/4" connector, and poking down through the floor hatch. Otherwise, it will fill the drain with liquid between dumps. I had that unpleasant surprise once. I used to put 5 gals. of water in it after dumping but now keep it empty except to travel and camp.