I used to have this problem and thought I’d have to replace the black tank valve, but I started backwashing the black valve surfaces by leaving the black valve open for a few seconds after opening the grey valve. This may or may not work for you depending on your valve configuration, but I haven't encountered a drop of yuck in the sewer hose connection (manifold?) since I started this practice. I could speculate on the types of stuff that could lodge in the valve mechanism preventing a seal, but suffice it to say, flushing the valve backwards with a good slug of soapy water from a full grey tank cleans the valve seating surfaces enough to allow it seal when closed.