My boat has a 30 gal. water tank under the port lazarette, and the fuel tank under the starboard lazarette. The boat originally had no waste holding tank, and a PO installed a Dometic Traveler head which has a 9-gal. holding tank mounted as a part of the unit. There is also a much larger what appears to be another water tank under the berth in the aft cabin. It is not connected to anything. Any one have a tank like this, and if so, what is it for? Thanks.