What you need is not a backflow preventer (check valve) as they commonly do not work after long term service. What is normally recommend is a ****vented loop**** , a discharge that goes approximately 3-4 ft. from the head/toilet straight up, then a vacuum breaker mounted at the top (will close when there is fluid being pumped, but will open when the pumping is stopped and allows air to enter) then a 3-4 ft vertical down pipe. ... thence to the holding tank. (Think of an inverted letter "U", with a air valve on the the top of the inverted "U"). Without the vented loop you risk siphoning the holding tank contents back through the toilet.... or risk sinking the boat if you are pumping overboard. Any time you have a discharge plumbing device overboard or into a tank you SHOULD discharge through a vented loop or you risk siphoning backwards to those areas below the water line of the boat.
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