Since I had to repair two of the underbelly plastic panels that fell off and dragged beneath my 2008 Silver Back during my last camping trip, I had the opportunity to address a leaky grey water valve and a hard-to-move black water valve. In replacing both valves, I found ABS cement in the gate of each valve. The grey water valve itself was worn out of it's track. The black water valve was "glued in." In following the instructions for replacing the valves, I noted that the factory did not allow a 24-inch radius between the valve and the handle, thus making the cable bend more sharply than it should have. I'm taking the time now to reroute the cables. Frankly, I'm very disappointed to see the "mess of cables and wires" hidden under the panels.
Your grey and black waste valves should not leak or be hard to open/close. If you can't install secondary valves as the others did, I'd replace the valves.
2008 Silverback 30LSA
2011 F-250 SuperDuty 6.7L Diesel