There has been a discussion on this problem before Gene. You can drill a hole in the center and then cut a slot being careful not to get into the threads of the heater. Might want to do a search in the search box to see if it will bring answers back up.
If you broke the head off there is a whole now. Cut a slot in the plug with a hack saw blade. Try not to hit the threads. If you nick them a little it will be OK. If you nick them alot use a chasher on them. After you have cut almost all the way through see if you can grab the side of the plug with long nose pliers and peal it like a can of sardines. This usually works very well. Replace it with a anode rod and you should be all set.
I purchased a large "Easy Out" designed for lawn sprinkler heads from Lowes. It will allow you to back out that plastic plug easily. Do not replace it with an anode rod. The Atwood company says that will void your warranty and is not needed as the tank is aluminum lined. You can purchase the plastic (I think they are actually nylon) plugs at just about any RV store. I always carry a couple of them with me just in case.