Eliminate the vac breaker valve if its somewhere in the rig, its a recipe for disaster. When you have low water pressure and use that tank flush the low pressure may not be enough to close the valve and the water will pour out the vent and flood your rig.
The poop police will not arrest you nor will plumbing code enforcement come and confiscate your rig. The possibility of that tank flush system backflowing into the park system is almost 0, as a specific sequence of events would need to take place while you're actually flushing the tank for that to happen. Just don't keep the fill port connected all the time, only when you want to flush the tank.