RE: Where is the pump for the drinking water?
If a US model Westy it's inside the water tank.  If a Canadian version it is
outside the tank, under the sink.
Access on the US models is from the top of the water tank.  Access to the
top of the tank is from under a tin cover, held with a couple of screws,
that is the bottom of the flat storage area, the rear storage area of the
two between the stove and the hanging closet.  Remove the tin plate and the
lid to the interior of the tank is obvious.
BTW, it's Westfalia, not Westphalia
Good luck