This is a common problem. I have heard of many trailers, including mine, that come from the factory with the pump input valve set to the "Winterizing" position. I was lucky to just stumble across the valve while troubleshooting this problem. Glad it was an easy fix for us. Wish the factory would reverse this valve before shipment.
Jim and Connie w/3 lb. Chihuahua= Nugget
2016 Rockwood Ultra-Lite 2608WS
2004 Dodge Ram 2500