You're going to have to trace the circuit then. I'd start at the front, where the positive feed comes from the trailer plug. As I said, on my trailer, there's a junction block on the street side of the battery compartment. You could see if you have +12V at that point with your Tow Vehicle's running lights on. If you do, go to the closest running light and remove it from the trailer. Test for 12V there. At that point, see if the ground wire is making good contact too. Continue this way until you find the problem.
However, I just had a thought. In my trailer the feed for the +12V and the ground wire run along the floor of the bottom half to the rear of the trailer. Then, they go up through a cloth covered tube in the bathroom to the top half. All the wires are spliced by wire nuts inside that cloth tube (so that the top half can be separated from the bottom half). It's possible the wire nuts for the positive feed or the ground wire have worked loose there. (Your wire bundle from the bottom to the top may be in a different place and the cloth tube may not be installed.) You can test for 12V at the wire nut location. If no voltage, the break is somewhere in the bottom half.
In mine, a mouse or rat had chewed through one of the wires in the bottom half, and it disabled a stop/turn signal.
- Jack