The subject of dumping a black tank has been discussed on here before, in depth. You can dump it wherever you dump your grey! They both go into the sewer. So in answer to your question it is easy.
Sorry but that's not correct on some sites the grey goes into a soak away which you should not put black waste down. This was the case at the Antwerp site we were at a few days ago, the grey was simply a road drain at the back of the site with a sign in several languages saying grey only.