A pressure regulator belongs at the water source before pressure reaches the trailer hose/plumbing chain.
READING GUIDERV WATER BASICS
What Water Pressure Is Safe for an RV?
Why campground water pressure varies and why a regulator is part of a basic travel-trailer water setup.
There is no single safe PSI for every RV plumbing system. Use the trailer or plumbing-component guidance when available and regulate campground pressure rather than assuming the pedestal is safe.
What matters in practice
Adjustable regulators are useful when you want to see incoming pressure and tune it conservatively.
Pressure control does not replace checking hoses, fittings and plumbing for leaks.