Manufacturers almost never supply a water pressure regulator on a motorhome simply because without it (but with a caution somewhere in the manuals to keep pressures low), they can avoid all warranty responsibility by claiming it must have been the owner's fault. If you never connect to city water, you don't have to worry about over-pressure situations, because the electric water pumps supplied by motorhome manufacturers almost never fail by over-pressurizing water from the tank. They usually have a hard time just supplying the 40 PSI or so that they are set to by their (usually fixed) internal pressure switches.
However, whenever you connect to city water, you are at the mercy of someone else's mechanical systems to keep the water pressure at a level where it will not destroy the various components of your fresh water system. A brand new motorhome usually has a marginal system installed to keep the weight and cost down, so when you add a couple years of jostling and chafing, an over-pressure situation is almost guaranteed to break something. And a good portion of your fresh water system cannot be inspected or replaced without a great deal of disassembly, being buried in the chassis with little or no thought of maintainability.
There are two types of pressure to worry about: dynamic and static. Dynamic pressure is the pressure present when water is flowing, and dynamic over-pressure is never a problem because when you open an end of the system pressures drop immediately, usually to a value too low. The diameter of a pressure regulator's diaphragm is a major determinant of how low the pressure will drop. Expect a large drop in dynamic pressure with one of the cheap $10 regulators (about an inch in diameter) from the camping store, a moderate drop a mid-sized regulator such as the Watts H560 is used (not recommended - see below), but only a minor drop if you are able to adapt a residential regulator (Watts/Wilkins 3/4" model) which has a much larger valve surface area inside of a bell housing. Picture
Static pressure is much more insidious, because all it takes is the tiniest of leaks inside the regulator to add some molecules of incompressible water to the circuit to drive the pressure up, when there is no place for the water to go to bleed off the pressure. The presence of high pressure for hour after hour will stretch components until they eventually break, if it doesn't happen immediately. Even otherwise good hose connections will allow water to squeeze out under high pressure, generally defined as over 60 PSI. The manual may specify 80 PSI as a maximum, but that is too high to operate at for long periods of time, especially when you consider the pressure pulses that can arise from a number of circumstances.
Unfortunately, brand-new regulators are very often proven defective, because it takes so tiny of a defect for them to allow a minuscule flow of water beyond the pressure limit set. Always use a pressure gauge to monitor pressures after the regulator, and definitely get a gauge that has a "maximum pressure" red needle indicator. Always keep an eye on the maximum pressure needle . Our experience and that of others suggest that you should stay clear of anything but a top-notch, re-buildable residential regulator. We've had infant mortality problems with the Watts H560 ($45), the Wilkins BR4DU ($35) and Wilkins 70 ($40), but the slightly more expensive Wilkins 600 ($55) has worked as intended (finally!). Be aware that the better hardware retailers like Home Depot will refund or credit your money for a regulator that has failed as long as six months (or longer) after purchase, so always save your receipts.
