Most often the small regulators will reduce pressure and at the same time, sacrifice flow. Usually the lack of sufficient flow is blamed on lack of pressure. I personally as well as many others have swapped out these flow reducing pressure regulators for more substantial units that reduce pressure without restricting flow. Look for a unit that will supply at least several gallons per minute at a safe working pressure. Also check the flow at a static rate such as a supply that provides 50 psi along with a regulator set for 50 psi.
I have never heard of anyone disappointed with the installation of a Watts, whole house type regulator. They are available at your local big box home improvement store.
My last coach came with a Sureflow combination inlet and regulator that barely passed 1 gpm at 40 psi. Replacing it with the Watts in the picture eliminated all flow and pressure problems. The manufacturer switched to large regulators similar to the Watts for newer models.
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