I think plastic weld is the only method for a proper and permanent repair of a poly tank.
Look at cutting away the old fitting and installing a spin weld patch or fitting in it's place. Not sure if this is a DIY process or not. Requires a special driver for a router. Router spins the patch piece at high speed creating heat that welds the patch in place. Patch pieces/fittings are cheap but the needed tools aren't. Perhaps there's a local shop that can do it?
Maybe the best thing to do would be call one of the RV holding tank manufacturers like Icon Direct, Plastic-Mart or Tank Depot. They also have spin weld patches and fittings.
I have to wonder if you could do your own mechanical patch. May use a 1-1/2" or 2" brass flange so you can get a finger(s) inside to install a s/s washer and nut. Drill some additional holes around it. Cut away the old fitting and use a gasket between flange and tank. Water pressure would push tank wall against flange & gasket that would help seal it. If you it mess it up somehow tho., you could be looking at a new tank. Use a reducer bushing to get back down to 1/2".
