Black Tank Repair - GOOD NEWS !!!

I am not sure where your crack is but if it is on the top of the tank or not under pressure I have had good luck with Shoe Goo and proper surface preparation. There are two types of bonds, chemical bond and a mechanical bond. Plastics that are not affected by solvents can't be chemically bonded so the surface needs to be rough sanded thoroughly to prepare the surface so you can get a good mechanical bond. Once you sand and wipe the surface with alcohol or acetone you can "water break" the surface just to be sure the surface is properly prepared. If the water beads up the glue won't stick if the water breaks and goes into the rough surface the glue will adhere. Wipe off the excess water and solvent wipe again, the solvent will flash of quickly and the surface will be dry. I would spread the glue over the crack then add a layer of FG cloth and work more glue into it to thoroughly wet it out. I have tried plastic welding but did not have any luck with it holding. Good Luck on which ever method you use.