ABS cement, in small can works good for ABS tank repair. It's readily available at most any hardware store, Lowes, Home Depot, Menards. Get a small can of ABS cleaner to use first, then add a thin layer of the black cement to dried surface. Now cut a small patch of fiberglass screen (like in your RV screen door) and press/apply it to the wet ABS cement, let dry. I use 3-5 more thin coats to cover screen, and beyond. Repair will look good, last forever, and is easy to use on a non-level surface. I've never had a failure, using this method.
Cement dries quickly, so have your screen cut, a bit oversize, ready to go.
Jerry