Good MorningMany years ago my wife and I owned a 1976 25' O'day. If memory serves me correctly, the tank was a small 5 gallon, located under the "V" berth. Obviously, 5 gallons is less than adequate if you are really going to sail. However, there is ample room in that location to secure an appropriate size tank in that area. From there, you could run the hose topside, via an access point on the port side. (I believe that the head is on the port side). Good Luck!