Cabin water pump is sending water gushing out fresh water inlet at the post and not thru the cabin faucets. Pump is working but sending water out the inlet.
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