The solution (one of them) has been located.
What I need is one of these
followed by one of these
That just leaves the pump. I think I will need one of these
The above thingy looks pretty much like the very old version of the one already in the ship. So I may be onto something. The pump would pull from the tanks and then push through the filters.
Finally, the sewage solution is likely to be one of these
or perhaps two (one for each tank). But lets start with one for now.
All in all the whole set comes in around less than what I was looking at for a single water tank improvement.
The water system will be pushing through at mains pressure and good flow rate.
So that job is all but done.