Yes, the city water hookup has a device in it to prevent the pump from pushing water out the city water hookup.
What happens is when winterizing the trailer, people pump antifreeze through there pump, then press the little "think" in the city water inlet hoping some antifreeze comes out. (I do it every year)
What happens is there is a spring loaded plastic piece with an O ring on it, and the O ring moves.
If you remove it from the camper, look down inside it from the city water side you will see a plastic piece that has another plastic piece passing through the middle of it. I can't remember if I drive the center out, but they do come apart (only press fit in)
Once apart you will see a valve, spring and O ring. Put the O ring back around the valve and put it back together. Then press the outer plastic piece that holds it all together with a small socket and a light tap from a hammer.
Or just buy a new inlet for around $15. 00 or so. If you go that route, check to see if your inlet (on the inside) has male, or female threads, both are just as common at the parts store.
All the replacments are brass that I have seen, which tend to not make that "squealing" noise when your running on city water, and somebuddy inside turns on the water.