While I don't know your exact model, I'd have to think its directly opposite your flush fitting on the outside. The vacuum breaker has to be higher than the hose fitting, so any water in the line will flow back to your black tank once water pressure is removed and not out the hose fitting, potentially contaminating the water supply.
Hopefully someone with your exact model will reply, but in the interim, take a peak inside your trailer, opposite your black tank flush hose connection...
Looking at pictures online of your model, this fitting may be inside/behind the cabinets between your slide and the bunks. Maybe there is a panel in the back of that cabinet that's removable?
Mike & Theresa, Madison, WI
2012 Jayco White Hawk 26SRK
2012 Ford F-150 EcoBoost FX4