That was the problem!
The connector from the trailer had a little mud in some of the holes. I took a screwdriver, took the crap out, and now it works like a charm.
Maybe I'll use a sandwich bag or plastic grocery bag wrapped with a rubber band to keep it clean from now on.
The mystery to me is why it worked on my way TO the campsite, when the trailer was parked on dirt at my house, and didn't work on my way FROM the PAVED campsite.