The pumps do lose their prime when run dry. I had this happen once or twice in an older MH and sometimes it took several minutes of running for the pump to prime itself. You may want to try using a jug of water with your winterizing hose to test the pump, at least you would know it is still working.
Stewart, Brenda and kids
2008 Newmar Canyon Star 3410, now at a new home
2006 Roadtrek Versatile 210