Get one of these,
https://www.amazon.com/Valterra-T58-.../dp/B000BGHYJS and install it to your waste hose fitting before connecting your hose. Once you have drained your black tank, close this valve but leave the tank valve open. Fill your black tank with water 1/2 full and add this product
https://www.amazon.com/Caravan-overn...9ZNM6KSN6DY9G3 which is supposed to clean your tank level sensors but will also dissolve tissue which is what is probably keeping your valve from closing all the way. This is way cheaper than having someone take apart your drain valve to unclog it, and a lot less messy. I use one of these valves, and if I find my black tank valve will not close all the way when draining the tank, I close the twist on valve and then with the black tank valve still open, I open the grey tank, which will back flow into the black tank and almost 100% of the time, will remove the tissue from the valve.