There may or may not be one built into your trailer, but in either case it won't hurt to put one right at the RV park faucet when you connect the hose. You can get them at any RV dealer and even at walmart for under $10. I would say worth it for the added peace of mind.
Also it will protect your hose, which of course the one in the trailer - if there is one - cannot.
Further, if there is one in the trailer, it might not even be working. If you buy one to install at the tap, for a few $ more, you can get one with a downstream pressure gauge that shows the pressure after the valve has reduced it, so yo know that it is functioning correctly.
Even with a regulator, probably a good idea to shut off the water at the park tap when leaving the trailer - if something should fail inside the trailer while you are away, you could have expensive water damage. Good advice - I really should fallout myself!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.