We have four tubes in and out of our FW tank.
1. Filler tube (~1/2” vinyl w/ corded reinforced)
2. Pump Tube (~1/2” vinyl w/ corded reinforced)
3. Over flow/ vent (~1/2” clear plastic tube)
4. Drain Tube (~1/2” clear plastic tube)
We have one additional tube; a clear ½” plastic tube used for winterizing. It is tied to the inlet side of the pump, with two valves, one valve to isolate the FW tank and one to isolate the winterizing tube.
Before tearing anything apart, check your winterizing bypass valves. You may have one not fully closed or that has a bad seal. You could grab a gallon jug filled with water and stick the winterizing tube in the jug and see if you are able to prime the pump. I would turn the valves a few times to times to ensure no debris got stick in there and not allowing a good seal. If that does not help locate your issue, remove the clear filter screen, housing on the inlet side of the pump. Clean the screen, check the O-ring, and the housing for cracks. Ours was full of spider cracks during the middle of the first summer on our HTT and it had to be replaced.
If all that does not locate the issue, I would check the fittings are loose or damaged.