As most have said, I use wheel chocks front and back regardless of the grade and put them down before I unhitch. I have seen the trailer slide of the front landing jacks when the surface is gravel. With my Reese elite hitch, you often have to back the truck up just a tad, to take the pressure off the hitch lock so you open it manually and the chocks keep the trailer from moving (yes, it will slide even on cement pads). The reverse is true when hooking up, you often have to pull forward a tad until you hear the "click" indicating the hitch is fully locked. Same deal, chocks keep trailer from pulling forward.