Marine heads have a very bad rap for draining back into themselves, leaving you with a smelly bowl of murky looking water.
The problems lay with two things, the joker valve and the oneway valve, the later gets less commentary than the joker valve and perhaps that is just to do with the name :-).
But the bulk of your fluid that remains in the head pipe up to your blackwater tank is held in place, hopefully, by your oneway valve.
This is a simple flapper valve and much more reliable than the very poorly designed joker valve, but it relies on pressure from the fluid above to hold the valve closed and therefore not leak.
If you allow your oneway valves to lay flat, as they were on our L450S when we first got her, they tend to not have enough pressure to keep them closed and they leak. So apart from a little olive oil helping to lubricate them from time to time…
One-way valves between the joker valve and black water tank are not common. I rarely see them.
A well designed one-way valve does not depend on gravity to be effective.
If the joker valve is not adequate to eliminate back flow into the toilet bowl than either the joker valve has not been maintained or there is a system design problem.
In my years as a naval architect and marine engineer working ship design, construction, operation, and SLEP I can’t think of an application for a one-way valve.
Thank you Dave, you raise some interesting points