I had the exact same thing happen on a PW 2008 Excel this fall. My valve was threaded into the opening on the rear side of the tank and I was able to screw the remaining piece out. I also noticed the plug (?) in the center bottom of the tank but did not investigate it any further.
My local RV dealer is closed for the season. Camco has a replacement valve. I'm toying with the idea of extending the drain line so I don't have to crawl underneath to reach the valve. I did find a appropriate threaded fitting at the local Home Depot which will allow me to do so.
Bob R.