Phil,
What do you suppose made the crack? Perhaps some stress from the camper being older and more prone to flexing while underway? Any sort of repair from a plastic weld to some patch or JB Weld, even a replacement tank may not be able to last long if there's flexing involved on a regular basis.

A true (professionally done, the entire thickness of the crack, not just a plastic outer 'bandaid') plastic weld would work but it, and most any repair mentioned requires the removal of the tank anyway so you might as well replace the tank at that point and save on labor. Plastic eventually gets brittle. Even if you have to use a somewhat smaller tank and re-plumb a few hoses, especially if you have the flexing, this could be a longer lasting fix albeit, at the sacrifice of a few gallons capacity.

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