Grew up in the pipefitting industry...other than ride-along, didn't do much RVng as a kid. Fast forward to my 1st RV (circa 2002 or so). The salesman (I know...) says hey, you need to watch out for water pressure in campgrounds. OK, makes sense to me. Salesman then says "We can sell you this pressure regulator" and showed me what was probably a perfectly good regulator....but I said, no, that's not a water pressure regulator. Water pressure regulators are red (possibly black) and say "Watts" on them and come with (as noted above) the oil or glycerin filled gauge and if you really get the right one, it will have threaded outlets for two gauges and an adjustment bolt.
Got a Watts but based on forgetting it a time or two or just being lazy, the ones from the RV places worked fine.