Ok that is norhing more than a spring chck valve... IT IS NEEDD.
What I would do (Actually did since I was not sure what was inside my rig) is go to ye old hardware and get two things, both brass, one is a "Spring check" valve, the other a vacuum breaker
Hose---Vacuum breaker--Spring check--Flusher inlet
When you are not flushing (Before you dump the tank PLUG the inlet
Or, get a new inlet, standard RV city water inlet, not that expensive.
Mine uses a vacuum breaker inside the RV, thus when I unhook a gush of water comes flying back at me, but it's all fresh water.
Home is where I park it!