Maintaining a gray water tank is much more difficult than maintaining a black water tank...'cuz galley sink water includes little meat and veggie particles that take weeks to dissolve which results sludge...cooking oils and grease...head sinks and shower water is full of body oils and bacteria.... So if you don't like maintaining a black water tank, you're gonna dislike maintaining a gray water even more.

The main problem with composters is liquids...not only urine, but the liquids in solid waste, which is 75% liquid. Liquids must be drained off the solid waste 'cuz wet soggy material doesn't compost, it rots and stinks...but they can't be legally drained or poured overboard inside the "3 mile limit," so they must be stored aboard and carried ashore...which IMO doesn't offer much advantage over a portapotty.

It's actually pretty easy to maintain a black water tank...just make sure it's well ventilated--'cuz oxygen is the key to odor elimination...and use an organic or bacteria tank product works with the naturally occuring bacteria in waste instead of against nature as chemicals do.