Tony,
This forum is principally concerned with the Fiat Ducato base vehicle. You would probably get more specific advice on a dedicated motorcaravan forum such as outandaboutlive.
Also I am guilty of originating a recent thread about forum etiquette, where I expressed a reluctance to answer threads giving thanks in advance, due to the implied reluctance of the originator to report the outcome.
Oh dear!
Does your vehicle have microswitches on the taps? If so the pump could be a submersible, inside the tank.
Please prove me wrong above, and let us know where you find the pump.