Crack in black tank? Is it ABS or Polyethylene?

My black tank had that same (I assume) yellow glue on it when it cracked. I used Plasti-Mend on it and it has held up fine.

As suggested take a few shavings off your existing tank and see if it dissolve in either Acetone or in ABS glue. There should be plenty of excess tank material you can shave off without danger of making a leak from the flanges where it is bolted up.