The early 25's have the water tank under the V berth on Port side. The ones which have the holding tank aft under the splash well, have the water tank forward under the V berth on Port side. The 1995 25's had heads which were optional. In the 2006 era on---not exactly certain where the change was made. From mid year had the self Contained 9 Gallon holding tanks under the commode itself and then the water tank was moved aft under ths splash well (My 2006 25 had that aft water tank) I would be possible to put one 10 gallon saddle tank on each side of the cockpit, without loosing any functional space.
Moeller makes a number of boat water tanks, but there are a handfull of polypropylene. water tank manufacturers. There is one made for Catalina yachts (saiboats) which may fit, as well as other builders. You have to gain access and then measure how much room, and what the angle of the bottom/taper/ height of the tank. I found several in the 25 gallon size available. NWMI may have records of the older boats or current boats which have the proper size. Request a cataloge or go on line to find all tanks that Moeller has patterns for. I have purchased Moeller tanks thru the years and never had a problem.
The tank, fill and vent usually are on the port side under the berth to balance the Helmsman's weight. on the starboad side. If you get a tank under the center it can be filled from either side. Vent is usually on the same side as the deck fill is. Tanks made in the UK and Europe may be prohibitly expensive for the shipping (and Tariff?)