On my 390CPMY I have the 70 gallon potable water tank under the aft bed. It is sort square and approx 48” long. Appears to be more room for a larger tank. I saw the inside of the tank from the inspection plate and it looks like a science project. I use bleach each time I fill it, and use it for showers and washing dishes only when we are on the hook or a mooring. Has anyone replaced their tank with a poly one? If so, where did you get it. I would like to get a 100 gallon tank, same size but longer to fit in there. We have 2.5 days worth of water for myself and the bride, but wish to be able to stay 3 or 4 days. I just bought a water level gauge with a tank sending unit, so I will have a better idea of where I am at use wise. Any info will help, as I have the entire winter to order one and have it made. Thank you all again!