I'd take a volt meter and test for voltage on the end of the cable coming from the pedestal, if power is there, your problem is located, if not, with the cable still disconnected, test for voltage at your jack on the trailer. While the A/C tester shown above will determine status of your pedestal, it won't tell you if voltage is coming down the cable. Test at the trailer with and without your TV/amplifier on.
IF I had to guess, without knowing anything else, I'd think the tv cable MAY be running along side (parallel/in contact with) the buried electrical in the campground.
I made the same mistake when building my garage. I ran satellite tv coax in the same 2" PVC as power to a detached garage. This run was for about 50' and the caused the ends of the coax to be hot electrically. Did a little reading and found that not to be sound electrical practice and re-routed it well away from power. Problem solved.
Good luck and hope you find your problem.
Mike & Theresa, Madison, WI
2012 Jayco White Hawk 26SRK
2012 Ford F-150 EcoBoost FX4