If the tank is black it is likely ABS, (most black/grey water tanks are ABS) Off white is usually polyethalene which needs spin welds.
If it is ABS and the pipe is black ABS (likely) you may be able to cut and replace the broken section with ABS pipe from hardware stores. They use the same ABS pipe used in houses. And since the pipe isn't under pressure if it is a narrow crack grind up some section of ABS pipe, mix it with ABS cement, and fill the crack. it will set and harden quickly. if he crack is in the tank itself, if you can find a section of flat ABS you could use ABS cement to solvent weld it to the bottom of the tank for at least a temporary fix.