The point at which you want to regulate pressure is at the city hose connection. Your hose is the most venerable to over pressure.
If what I think you are using at the city connection, one of those brass 1 1/8 dia. 3 in. long so called regulator that is a complete waste of time and money. I have never figured out how they have gotten away so long with calling them regulators since you can blow air through them without restriction. At best that could be a Volume Restricter. If you want to regulate at the city connection you need a diaphragm type pressure regulator.
As for low pressure in the trailer. Question. Is there pressure there when you first turn on the faucet and then dropping off quite quickly. If so my first point to look at is the screen at the trailer hose connection. They tend to get cover with debris from the city system. Check and clean if needed.
If you have low pressure at the moment you turn on the faucet you may have a regulator in the trailer that is set low. Not that likely but that would be the first thing to look at if the pressure is low at the start of flow.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles