Hi,
The fitting is probably spun welded to the tank. I had the same problem with my black water tank at the three inch drain fitting. I had to replace the tank.
i might suggest you look at https://www.recpro.com/universal-rv-holding-tank-installation-fittings-kit/. These fittings are installed from the outside of the tank and work quite well. If you have access to the leaky fitting and if there is enough land, perhaps cut out the leaking fitting using hole saw and replace with a larger size one. Then you can plug the new fitting.
good luck.
steve
