A water regulator goes into your water supply line, and prevents pressure in the RV from exceeding its set value. They are available in fixed and variable. Camping World has them or at least the fixed ones.
They are usually a brass tube about 4 inches long, with a male hose fitting on one end and a female hose fitting on the other.
They can go anywhere, from directly at the water source, somewhere in the middle of the hose, right at the entrance to the RV, or even built into the RV.
Fixed, at around 40 to 50 PSI, is fine for RV usage. I think many water sources are 80 PSI or more, and the hose can act as an 'amplifier', particulary if the sun shines on it.
Note that not using one of these will probably cause you problems. I didn't use one 'just for an hour' and blew the pressure sensor on my water pump and the supply line to the toilet.
You definitely want to attach a regulator to your hose due to the high preesures at most campsites. I bought one today at Wal Mart which it factory set 40-50 psi. All it does is screw into your city water inlet and then attach the water supply hose to it and you are ready. Around here the brass reulator at Wal mart was $8.00 and the plastic version is $5.00. I would strongly suggest the brass one for longevity and durability. Just my two cents worth .
The plastic one from Wal-Mart usally lasts about 1 day before the pressure blows holes in it. I just read in the Good Sam HighWays magazine that turning the tap down at your RV Park site water source to lower the pressure doesn't work. All it does is lower the volume of water. As soon as you turn off all taps in your RV the pressure goes up to the RV Park pressure, usally high, regardless of where you set the water source tap.
You do want to get a quality pressure regulator because the cheap ones do fail and they sometimes fail open. The good ones are designed so that if they fail, they fail closed and protect your water system. Also the cheap ones have only about 1/2 the diameter of water passage that the better ones have and so restrict the volume of water that you can then use.
The regulator should go between your white water hose and the hydrant so that it also protects the hose from breaking . RV water systems are designed and tested to a max. of 60# and it isn't at all unusual to see much higher pressures in some cities. We are presently in San Antonio and I keep a pressure guage on the inlet and another on the outlet of my pressure regulator. The inlet side is presently reading 165# so there is little doubth that the water systems would fail if there were no regulator.