It does not work in the Black tank, TheFitRV did a really good video on it a while back, look for it on Youtube. Not sure it would help your grey tank either. If it needs a really good cleaning, try using dishwasher detergent, go for a drive, than dump. It does not bubble, and it is designed to break down the type of dish related grease and food particles that end up in the grey tank.
Rob
rvtechwithrvrob.com