Everything but the kitchen sink?
Nope! Even that is thrown in.
Apart from Jacques Tati's Renault 4 based camper in the film Traffic, and maybe one or two of those custom built Maharaja jobs, which manufacturer offered a model with built in sink with hot and cold water on tap?
The Mini Clubman Estate "Dormobile" I designed as a teenager had a sink and taps. Of course, I didn't actually build one, but I did some quite detailed drawings of what I would have done (wonder where they are now). You'll have to take my word for it, but it would have been amazing, if a little cramped.
Not sure exactly when James was a teenager, but I suspect Jacques Tati beat you to it. If you have not seen Traffic get yo' ass down to video-ezy right away, not only a great movie, but some cool cars, and the star shows off his camper conversion based on a Renault 4 van; topical too as C&SC recently did a story on the 4.
Meanwhile, due to total lack of interest in the subject, and I doubt anyone will show any interest this side of Halley's Comet anyway, I may as well come clean here and now and tell anyone out there still hoping for an answer, the Horch V8 930V shown at the 1939 Berlin Motor Show (I bet that was a fun do!) had a wash basin that folded out from the front wing.
End of story, 'bye.