I think that specific photo came from Adventure Trailers: http://adventuretrailers.com/accshower.html
I've looked at these heaters to replace the flat plate model with more hot water, maybe something to do this summer. The 10 liter version would fit between the frame rail and the outside, just like the batteries, but on the other side of the van.
I was looking at one of the Isotemp Slim models, http://www.defender.com/waterheater.jsp
that holds more water, is made of stainless steel and has a 120v heater option, but might be too large to make fit into this space
Please post what you end up doing, or if you want to do a joint order from Europe.