Not enough information, but, sounds like your plug was plastic and you broke of the part of the plug that the wrench catches? So the threaded part of the plug is still in the tank and now there is no where to unscrew it?
If so, just go to any Home Depot type of store and get an easy-out. The plug is plastic, so it should drill in very easy (not like a metal broken bolt). And the easy out should just continue unscrewing the broken plug.
If this is what happened, its really easy to get that plug out, especially if the plug is plastic.
Suggestion, when you replace it, go to Home Depot or Lowes and get a PVC version rather than the plastic one from the camper dealer store. And then... remove it often and don't tighten so much, replace every year too.