Lots of good info by others here. Had similar thing happen. The plug just decided it was going to leak nd when I tried to turn it out the top broke off leaving most of it in the tank.
A couple of other RVers came to aid. Eventually cut with knife and a screwdriver and got it out. It is a tough material to cut with a knife.
We damaged a few threads. Atwood Rep was there next day and used a brass fitting to retap.
I now have several spares but the easy out tool is something I've let slip. It's on the shopping list.
Replace the plug every year and use enough teflon tape and don't over tighten. Don't use metal even with teflon tape unless you want to. i wouldn't galvanic action is assured.