Ditto Iraqvet05 comment. Many TV's have a sensor to turn on the running lights when it gets dark. There is usually a manual switch to turn them on yourself. Use the manual switch to turn on the TV running lights and then check the lights on the trailer.
Our running lights went dark during a recent trip. It turns out a screw that butted up against a running light wire worked it's way through the insulation and shorted the running lights. The trailer fuse in the truck blew. Once the short was correct and a new fuse installed the lights worked normally.