Just a guess, but there could be something wrong with the tank vent.
When simply emptying the tank, air is drawn into the tank from the roof vent.
When you are flushing the tank and the tank valve is closed, water is displacing the air in the tank. Typically, the air goes up and out the roof vent. In your case, maybe that air is leaking from the system and finding it's way into the coach.
A simple place to start is the mushroom cap vents on the roof. Take a screwdriver up with you and take the cap off the vent. Look down inside and see if the 1-1/2" vent pipe has slipped down into the roof or if it still meets the vent cap.