One of the most frequent reasons for valve leakage is solid material, toilet paper, feminine products or baby wipes that get stuck in the rubber seal. That causes the valve not to seat properly and leakage occurs. Without knowing how you use your black tank, that may or may not be an issue. Essentially, the only thing we put into our black tank is single ply toilet paper (that breaks down readily) and human waste. Otherwise, everything goes into the trash can in the bathroom.
We also have a "terminal valve" on the outlet where you keep getting splashed. You will find that having an extra valve to open/close gives you that "insurance against splashes" and is a good investment. Here is a picture of the valve we installed and the link to one source for the valve. Installation is simple, just remove the cap, install the valve and reinstall the cap on the new valve.