There is a check valve in that connection. They are just under an inch long, and often people do not even recognize them. I might hazard a guess that someone did not blow that line out when winterizing, and there might be a tiny crack in the check valve that leaks down onto the black tank.
Take the screws off and the sealant, and pull out the water inlet screw-in connection, remove the check valve and replace it. They are cheap, less than $10.
One way of checking this is to disconnect from the water screw-in fitting, and try your pump. Leave the pressure on and see if the water leaks still.
It's worth just changing it.