leaking holding tank
I would be most hesitant to try and repair a polyethlene tank that has a crack in it. It is highly unlikely that you will be able to actually repair the cracks and you will find that virtually nothing will seal the cracks for long.
I have had the same problem with my water tank in the nose of my Morgan 33 OI...it has a leak around one of the top fittings.... leaks whenever the water sloshes.....and I have put just about every adhesive known to man on the leak....none have worked. The only thing that has stopped the leak has been to drain the water tank. I''m currentlly getting ready to replace it.
Given the potential health risks that go along with a holding tank leaking or a fitting breaking off and the tak draining into your boat and the bilge......I would think it would be in your best interest to
replace the tank asap...even if it is difficult and inconvenient.