FR had a bunch of vacuum breaker/check valves on those systems that were bad. Somewhere in your rig, maybe in a closet you should find a couple of tubes that come up into and out of a small black box. This is the the vacuum breaker. You can verify if it is bad by disconnecting the tubes from it, have someone turn the water on for the "sewer flush" port, you will get water out of one of them really fast. Be sure the person outside at the water valve can hear you yell "turn it off" so you wont get a flood, have towels and a bucket handy. If you get water then more than likely the check valve is bad.