I still don't understand what the park did? You said they installed an anti-backflow valve on your black tank flush hose? Then you said YOU turned on the water, meaning the water was turned off. Why would they have disconnected your fresh water hose and hooked up the flush hose? If you have a Y connector past their water valve and you leave it hooked up to both the fresh water connection and the black tank flush valve, you were just asking for a problem anyway. I wouldn't be surprised if the park's insurer denied your claim by saying it was your negligence for keeping the flush hose connected and you admit to turning the water on.