We have a system something like you describe. Sink, washbasin and shower all drain by gravity into a 'sump' box low down in bilge. This is an off-the-shelf unit made by Whale for the purpose. It holds a couple of litres only. Grey water is pumped from here into a flexible Grey Water Holding Tank. The grey water from this tank is used to flush our Lavac loo so that we do not have any salt water to clog up the pipes leading to and from the loo - a common problem. Outlet from loo goes to Black Water holding tank which lasts us about 3 days before it needs pumping out.

Does it work? Yes, provided we look after it all. The only problem is balancing the amount of water in each part of the system - running out of flushing water at the wrong moment can be unpleasant! The other problem is that the grey flushing water can pong a bit in hot weather. We cure this by putting a 'blue brick' in the sump box. This gives us sweet smelling blue flushing water just like at home!

Have fun.

David