If you are set on repairing, rather than replace the tank, you could use ABS Cement.
I have used this method on previous trailers, and never had a failure.
You would have to dry and clean the damaged area, then use ABS pipe cleaner, from any hardware store. Then you need a piece of fiberglass screen, similar to what is in your RV screen door. Cut the screen to a larger size than the actual break.
Now apply ABS black cement to the clean surface, and quickly press screen into the wet cement to cover the damage, plus enough screen, and cement, to attach to the good area.
You need to let dry, and then add three to five more thin layers of just black ABS cement, letting each thin layer dry before adding the next.
Jerry