cabin water pump

some folks warn that what I'm about to tell you will break the inlet.> Well. it's broken already so what do you have to lose? (A dry shirt)

Remove the screen washer (Get a new one at the hardware as it likely can stand to be replaced. or reuse it)

There is a little usually white "Button" behind it.
With water pump on push button
You will get wet
IF something is jamming the valve it may flow out.
Or not

Connect your water hose to the inlet and turn it on for a minute or so. The check valve is stuck and running water through it, as designed, may close it and knock the debris loose that is holding it open.

Your city water hookup has a check valve inside which allows the water to flow IN and not OUT. That valve is failed, failing and/or is stuck. Whatever the cause, it needs to be replaced so as not to fail again in the very near future. Easy to replace and pretty cheap. Less than $20 buck either from Amazon or Camping World. I've also seen them in the RV section at your local Wally World....Dennis

Needs to be a water hose plug as 3/4 pipe and water hose is different threads.

City Water hose connection....check valve has failed

Turn pump OFF
Remove Screened washer in hose connection
Then push in on the white center stub and see if it springs back
Sometimes they get stuck

But if that doesn't work....screw a 3/4" PVC plug in the hose connection
Or a garden hose shutoff valve
*Both at any hardware store

OR you can replace the whole city water assembly

Replace inlet, or easier, put a plug on it.