Sealants for a cracked black tank made of LLDPE

[Isaac-1]

A new 49 gallon black tank is 220.
Maybe 3 hours of labor to drop and replace.
One and done.
I think you'll fight this over and over until just replacing the tank becomes the only option.

Unfortunately in many locations that turns into $220 for the tank and $400 for shipping

 

[NY_Dutch]

I did find that I misspoke most black tanks are polyethylene not ABS.

And the LLDPE the OP's tank is made with is in the polyethylene family.