I thought this was the issue. I like taking simple measures. Do you have a meter or a test light? Look at this diagram..
Meter with Ohm or Continuity tester
Take Pin 1 and touch a good ground on the TV or Trailer. You should get Continuity. If you don't on either the TV or Trailer...or both, then I would look into where the wire is broke or just run a new ground.
Take the test lite to pin 4 and test both to pin 1 and to a good ground on both the TV and the trailer. See if the test light illuminates. If between pin 4 and 1, on either the TV or the trailer, the light does not illuminate, you found where your problem is.
That is my way of diagnosing. Everyone has their own method that works well for them.