Many of our vintage Airstreams have a "pressure regulator" in the circuit to protect the plumbing from high water pressure. (Airstreams like 40 psi water pressure.) The pressure regulator prevents high water pressure by making the water open a plunger with a spring behind it.
If your pressure regulator is stuck shut, then the water pressure can't open it and you don't get any water flow in your trailer's system.
This is one cause for lack of city water pressure and flow in your trailer.
You can verify this by adding water to your fresh water tank, turning on your 12v
water pump, and verifying you have water pressure and flow to your faucets in the trailer. Pressurizing your water system with the 12v
pump does not involve the city water pressure regulator.
Our old trailers were originally plumbed in copper. They are a bugger to work on. Replacing an old pressure regulator is a real job in copper. You will find the PEX pipe and fittings are your friends.
Let us know when you find the problem.