You need a pressure reducer, check valve, and Y fitting, the pressure reduce comes in two types, one type is inline for water hose connection. The other is bulkhead mount, I have the inline type - it lays up under the forward end of the gunnel rod storage area.
The check valve goes on the outlet out of your tank, and next down the road you add the Y fitting.
I think the West Marine catalog has a drawing of this layout.
