Check valve needed after all
Actually, what I found is that the brass Watts pressure reducer, installed by Airstream in my 2000 Bambi, has a brass check valve screwed into it. Removing the leaking pressure reducer removes the check valve, needed to keep water from leaking or flowing out of the city water connection if you are using the onboard tank and pump. I cut out the Watts unit/check valve assembly, and will replace it with a brass SharkBite check valve ($15). I'm installing a Camco brass pressure reducer ($9) at the city water inlet (located at my back bumper). The pressure reducer could be installed at a water spigot, instead, to protect any water supply hose as well as the onboard pex piping.
No WD, but a great DW!