Make sure you've really primed it well, and that the fresh water tank valve is closed. I have had the same problem after the fresh water tank has been empty and unused foe awhile. I pour maybe 2cups into the intake hose and run the pump for maybe 2seconds and repeat this until I get it pumping. It seems to need a good prime IF the pump hasn't drawn fresh water for awhile.
The first time I winterized, I had just used the fresh water tank and had no issues. I want to remember to put some water into the fresh water tank and use it before winterizing next year to see if that really is a solution. Good luck!
Jim & Kathy
2012 Crusader 290RLT TE, TrailAir flex pin: 2012 Toyota Tundra 5.7 SR5 Dbl cab 4X2 Tow, 4.30 gears, RideRite air, TRD cold-air induction & dual exhaust: Reese 16K Slider: TST tpms
Miles towed to date: 18,950 Nights camped 192