Black tank cracked...twice.

As for the repair, any chance you could slather epoxy, fiberglass resin or similar over the area rather than a second tank replacement? An initial thought for me was that some weight could have been placed on the drain outlet, causing damage where it connects to the tank. But the pipe's hanger would bear the brunt of that load, so I'd carefully check the hanger for evidence as well. Otherwise perhaps vacuum damage as others have mentioned. A unique and interesting scenario.