Sounds like the 'anti-siphon vacuum breaker' on the black tank flush system has failed and is allowing water to pour out top cap .......
DO you see something like this device in the bathroom sink cabinet?
Flush water comes in thru bottom line/connection then goes out to tank nozzle thru the side line/connection
Water pressure is suppose to close off top section which is open to atmosphere
If it doesn't then big leak source inside the RV