Years ago, When I had my first Travel Trailer it had a single axle. The door was toward the front. I unhooked it at the campground walked inside and toward the back. When I did the jack on the front of the trailer came off of the ground and the trailer started to roll down the hill toward the lake !!! I ran toward the door on the front of the trailer and the jack came back down digging into the dirt.
After I got it reset on the camping spot I drove to town and bought some wheel chocks.
It's funny now but, at the time it was scary and embarrassing.