There's often no easy fix to an external water pump problem. Eriba Tourings as standard, have under sink and basin water containers with pumps in. Having an external water source can be either a factory fit option, a later retro fit by a dealer or mobile tech, or a DIY job. That means that there's no consistent type or manufacture of systems/parts and without seeing and knowing how it's been put together it's hard to diagnose remotely. It should, obviously, work off the battery, but if DIY'd there's a possibility it may still be connected to the transformer that's used in non battery applications (again standard spec.) to step down EHU 240V to 12V for lights and pumps. If it is wired to the transformer it needs 240V to operate. That's just one option. I take it that the pump isn't operating on either the sink or wash basin, which rules out tap microswitch problems, unless both microswitches have failed, which would be a helluva coincidence. I've had a tiny bit of plastic break off the inside of an external socket and that's stopped the pump working, then there's fuses, dodgy connections etc. Good luck, but such an issue can be mystifying.