Hmmm, if I were you I''d start tracing out the plumbing and figuring out how it all works.
When you are unplugged from dock water you will need the electric pump to provide pressure for the water system. Start by checking if the tanks are full and whether the pump is functioning (sucking air, blown fuse, ???). Also check for a 3-way valve which might select between dock water and the onboard pump.