Hard to see from here, but for a repair that large I'd consider removing all the old excess glue (*if* you can) and what's left of the protrusion for the fitting and put a patch over it all. If you go to a local plastics shop you can get a piece of sheet ABS or find an online source. It's available in 1/16" to 1/4" like in
this website for example. ABS sheet can be bent/formed with a heat gun. To remove the old cement, try a heat gun to soften it and use a scraper, small sharp chisel and/or utility knife.
The Plasti-mend method might be the way to go but will it work over old ABS cement? If you can remove the old cement where you need to down to clean ABS, re-attach the old fitting with cement and then patch the missing/damaged portion with the Plasti-mend patch kit?
The original tank wall will flex somewhat when full and if travelling with contents in it. If putting a patch piece over it, iIt might be a good idea to avoid sharp corners and make the patch piece as rounded as possible to reduce stress points.
There's a number of RV holding tank manufacturers online that have fittings and info. There are some youtube vids showing how to do tank repairs that may or may not help much.
It would be nice to see some photos after the repair.
FWIW, are the tank sensors not installed at the recommended elevations? Looks as if there isn't much change in elevation between them. Not uncommon.
