If it has an air vent, which it should to vent sewer gas, I would definitely vent the black water tank to the outside. If the tank is store-bought (not DIY) there may be instructions that come with it. Everything else should be pretty straightforward. The toilet should have some kind of air trap to keep odors out of the van; if not, then install a U-curve in the drain to do that, such as what you see under a sink, but bigger, with the downstream side of it vented to the outside (air vent, above the level of the tank). Home type toilets have the air trap built-in, and yours may too judging by the picture.