Look for one of these.......could be clear plastic or even brass (black plastic common)
That is the Anti-siphon vacuum breaker check valve device
RV Flush System
Water supply goes into bottom of device and pushes a disc/flapper up closing off the air slots in top cap then flows out the side to nozzle in tank
Water turned off disc/flapper drop down opening the air slots breaking vacuum and allowing water to drain back into tank
If disc/flapper doesn't close off the air slots water pours out of them flooding the RV