Add an Access Opening?
Before installing the new one you might consider adding an access opening for future cleaning of the inside. The 'plastic' tank will last forever so at some point in the future someone, maybe you, might want to clean out all the junk in the tank. Adding an access opening now would make it a lot easier. If top access is difficult when installed then ideally the installation of the new tank should provide for removal for cleaning.What does the boat use for a waste holding tank? Were these aluminum too? If so, if the waste tank hasn't been replaced (and I'd be surprised if it hasn't) it probably won't be long until it needs to be. And on another note, if the water tank is aluminum I give some thought about whether one should drink the water. Even though the water is cool and not heated, it sits in there for a long time so there is a long duration contact with the aluminum. May want to use bottled water for coffee, tea, or cooking with food. We use bottled water just because the water tank water is stagnet and not changed out enough.Boats are a lifetime project! If it isn't one thing it's something else.