Seems to me that If you don't want to use raw water from outside the boat, then I think you are better off installing a second water tank for flushing and keeping your clean water system clean. No matter how many one way valves you install, seems like bacteria always manages to contaminate the water system when they have any connection. Probably because of the pitching and rolling of the boat and the lack of a fresh tank like in your home. How big is your holding tank? install a secondary water tank that is about 90% of that size. Keep a connection to outside water through a "Y" valve incase you can't get water.