Assuming your tank is like mine, the 2 inch pipe section, connecting the tank to
the Valterra drain gate, is polyethylene (or a similar material).

The corner edges of the tank look as if they were welded rather than molded. I
have no knowledge about welding polyethylene but if that isn't practicable you
might try using J.B. Weld to mend the crack.

Garrett Adams
83.5 Adventurewagen GL
Stockton, California