If the black tank gate valve is leaking water, then you have a mess inside the black tank. When last you dumped it, you probably got a little bit out by running water through the black tank flush. There's probably a bunch of solid waste in the tank and it's likely blocking the opening.
Get a clear sewer outlet adapter with a backflush fitting. Or get a
combo that includes an external gate valve. Or, if you get a clear adapter with backflush, but no gate valve, place a separate
twist-on gate valve after the clear adapter. Close the gate valve and attach a hose to the backflush fitting on the clear adapter. Run water for 60-90 seconds to push the blockage away from the black tank valve. Then turn off the water to the backflush fitting and see if you get a bunch of stuff coming out the sewer.
If that helps, then after it stops draining again, close the external gate valve and run the regular black tank flush for 6 minutes (timing on your phone). Dump the tank again. Use the clear adapter's backflush fitting if necessary to dislodge any additional black tank valve blockages.
Repeat the fill and dump cycle 3 or 4 or 5 times until everything is out of the black tank.