You did verify the fuses are all good for the trailer running and signal lights, right?

 

If gooseneck try putting grease on the ball that will give it a better ground.

 

i had a prob one time, hooked up and had no power to trailer, check under hood for blown fuses, all good. i had no choice but to splice into a good known power feed, 12volts for the time being. it turn out to be a broken power wire, in the stock harness right at the fuel tank area. i ended up splicing into that harness in front of the tank, and running a new wire to the other end of the fuel tank and splicing back into harness. when i pull my box off for paint this year, i will have a better look at the harness and fix properly to not have confusion with a mess of wires.i used a test light and probed the harnes every couple inches till i found power, working from the plug to wards the front of the truck.