If you can get to the cracked area, I think you can repair it (yourself). After cleaning the surface, paint the crack and the area around it with ABS plastic cement. (You can get this in Home Depot, Lowes, etc. Then with the cement still wet, push in sections of fiberglass cloth cut to cover the crack and the area around it. Follow with another coat of ABS cement to cover the fibertglass and bond it to the tank.
I had a crack around the collar where the drain pipe comes out of the black tank and this is the way I repaired it. In my case, after the first repair had cured for about 24 hours, I repeated the process with another coat of ABS cement and more fiberglass strips. It has not leaked at all since I made this repair.
The collar at the black tank drain is a weak area and cracks develop there due to minor flexing of the drain pipe against the tank. This may be where yours is cracked. It's really an easy fix.
- Jack