That's tricky. What I would do as a last chance before the dreaded dismantling is:
1. Close the grey tank outlet valve.Open the black valve fully, but install the end cap over the combined outlet so that the black tank can hold water by virtue of the end cap, not the valve.
2. Fill the black tank with water, while slowly adding a complete box of Ridex drain clearer. Leave for a day or two. This will enable the Ridex to eat any biological residue by the black valve.
3. Remove the end cap, and let the fluid rush out in one full blast into a sewer.
4. Repeat the procedure but with clean water without the Ridex, and dump immediately. Repeat if you have time and enthusiasm.
5. Test to see if the valve is any easier to close. If not, the rubber seal may have torn. It's worth a try. Good luck.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."