The discharge port (aka spud) is a hard place to fix a crack/leak. Apparently there is movement there that causes the crack. There is a ton of threads on people fixing, or attempting fixing, those tanks. A "Search" on keywords like black grey tank leak abs crack etc. should provide lots of interesting reading.
Some have reported success using a product named Plast-mend (I think it is on Amazon). Here is one (there are more) link to one members success using it.
https://www.montanaowners.com/forums...ad.php?t=79251