Many of the motorway service areas in Italy have dedicated areas for camper vans. This is the best I have seen so far. As you can see it has a special place for camper vans to park as well as a place to dump your grey water and to fill up with fresh water. It is clearly suggested that you can dump your black water here, not that I would hope anyone on this forum would do so although in other places I noticed evidence that other people may have done that. After you have dumped your grey water, the device gets sterilised. This one works, unfortunately most do not.