Years ago on the SKP forum I learned a trick with the gate valves that has served me well for both grey and black. On the plate just above the puller, drill a small hole. After emptying the tanks, use silicon spray, with the flexible nozzle, to spray inside the housings, both with the valve closed and open. Distribute the lube with a few pulls, and the valve is as free pulling as new. Put a screw in the hole (I use a faucet washer under the screw head to seal it). Four years old rig, 9 years 'new' rig always easy pulls. I need to do this about once a year. Works for me.
Gary & Eloise
'05 Allegro Bus, 40TSP
'01 Honda CRV; fulltime since 1991