This space saving 90 litre water tank fits above the right wheel well of any Fiat Ducato, Citroen Jumper, Peugeot Boxer or Ram Promaster built after 2006. Not sure if it fits other vans too. It comes with several prefabricated holes and connections: Two maintenance lids on the top, one inlet and one outlet. The outlet is 26mm pipe that is molded on the bottom of the tank, so you must drill a hole in your van and attach a ball valve on the outside.  

Unfortunately, it seems to be only available in Europe.

EU: https://www.campingwagner.de/product_info.php/info/p62200_Frischwassertank-fuer-Vans--90l.html