"William G. Andersen" wrote
the drain is blocked, obstructing a free flow of water to the bilge

That's a distinct possibility. That tube looks pretty big in the photos. It
could hold a fair amount of water if the back end of it is stopped up: When
the boat is floating the water could be stored in the tube but when it's on
the trailer it could flow forward into the floor. Fishing something through
the tube to see if it's blocked sounds like a good move.