If servicing the black tank system, I recommend first doing what I do at home at the end of every RV season.
Open the black tank valve so the cleaning process includes the gravity drain piping and macerator. Fill the black tank with hot water. While it is filling, add 1/3 gallon of standard laundry bleach. I have a service sink in my garage that I hook a garden hose to for hot water, and fill the black tank directly through the toilet. Screwed on the end of the garden hose, I use that common 18" white RV wand with the little yellow shut-off from Walmart. I fill the tank to within a few inches from the toilet flushing valve. I then run the macerator a little then quickly screw on the end cap. This allows the macerator system to be cleaned. After a 2 to 3 hour soak, I run the macerator for a minute, then let the rest rush out the gravity drain. I repeat the entire process without bleach because bleach will damage the system if left inside.
We don't do #2 in our toilet so the only solids discharged are remnants of toilet paper. I also do this to the gray tank. The reason I do this is because our PC is in our garage, located under our bedroom. It's all about eliminating odors. 12 years of this practice and our PC has never had valve issues, nor any odors inside the PC, nor in the garage.
I do wonder if you followed my process, just maybe your valve will clean up and work well again. The key is using "hot" water. I add the bleach primarily to disinfect the discharge for my local environment and make the system fresh like new. But the bleach might also help dissolve whatever is making your waste tank valve sticky. In your case, during the 2 to 3 hour soaking process, I would exercise the sticking valve so the hot bleach solution soak will be more effective.
Just my thoughts.
