Makes no sense to install it INSIDE the trailer. But you do need one. I agree with the others: put it on the city water spigot, then put your hose on the regulator.
I've had experiences with 3 places that had really high water pressure. The first one told us as we signed in that a regulator would be needed in his campground because the city insisted on supplying him with water at 150 psi, which would blow all your plumbing without it, and he told us to put it on the spigot. Second one said nothing, I put the regulator on the side of the trailer, and about 20 minutes later saw that my hose had ruptured. Third one also said nothing, and before hooking the TT up, I went to hose down the side due to having gotten splashed with mud, and with no nozzle on the hose it shot water a good 40 feet. needless to say, regulator went on the spigot and I saved a hose.