The sewer outlets are normally black ABS plastic. Not the same as PVC, so be sure to get an adhesive or sealer that works on ABS.
I used plumbing repair tape on a previous coach that developed a leak there. It's a self-vulcanizing, wrap around tape that bonds with itself to form a permanent fix. Some brands are Rescue Tape, Tommy Tape, ER Tape, etc.
Welcome to Rescue Tape!
Home centers and even Walmarts usually have it.
Another type of repair is an epoxy putty that you mold around the crack.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL