A post-script with a note on older Interstates...

Initially, I didn't think much about this issue other than I grabbed a regulator in a camping store just in case there was a question about it ever being needed for some reason. The 2007 owner's manual has the same advisory verbatim as what was quoted above for the 2014 manual. The parts manual doesn't supply any technical details other than to suggest that said regulator is integral to the fill apparatus installed in the side of the Interstate.

This week I saw this type of sign for the first time in a campground, so I inserted the external regulator ahead of the shore line. Even if it will not reduce static pressure for the Physics 101 reason given in the comments above, it's still nice to have some information regarding the stated pressure - it is really high such that I maybe ought to keep an eye on it, or is there just the potential for it being high?

In this case it proved to be a non-issue, but I think I'll leave the regulator attached to the line so that I can compare to future pressures.

Actually, there is.

I posted this some time ago. My 2011 leaked under high pressure at the plastic intake regulator. After removing the refrigerator, I found this:

https://gmillerpc.com/misc/2.jpg

All that is standing between you and a flood is a cheap, plastic regulator held together with four screws. That's it. In my case, the device leaked because the screws, over time, either loosened, or never had been, fully tightened. Didn't matter as the damage was done. But even without that issue, the regulator itself is simply not very robust. I wouldn't trust it.

Regardless of what Airstream suggests in the manual, using a quality (i.e., brass) regulator at the tap won't cause a problem under normal pressure. And, you won't have to rely on a very cheap and flimsy plastic regulator to keep from flooding your van. If it does cause pressue issues, you can simply remove it from the line. I'd rather be over-cautious than risk a major flood if the plastic regulator fails.

But if you come to a park that has extremely high pressure, you are rolling the dice that what happened to me will happen to you. If you want to stack the deck in your favor, use a separate regulator at the tap.

I remember that photo - part of this discussion is in another thread. The next time I bring our Interstate out of storage, I'm going to look at exactly what apparatus is associated with our city water port. When we did our black/grey water vent line repair almost a year ago, I noticed the incoming water line; in fact I took a Sharpie and marked something like "City Water In" on the inside of the bulkhead under the cabinetry, so I would not forget. But I didn't note whether it actually had any regulator-like device associated with it, or whether there was plastic vs. brass or some other material.