If anyond has done any recent wiring on your trailer, my first choice is someone connected a hot wire to ground.
If your TV lights dim, you don't have a bad ground. That's too much current being drawn and a bad ground doesn't allow current to flow. You have something shorted to ground putting large load on the 12V coming from the truck. You have a hot wire that has had the insulation worn through and is shorting against the trailer frame. You could probably figure out where it is, but un that time, you could pull a whole new wiring harness. Beside, if there's enough wear on one wire to cause a problem the others probably aren't far behind. I just rewired my trailer because it was a salt water rig with all of the problems associated with that. The wiring harness cost me $10 along with another $10 for white wire to do the grounds. I ran a hard wire grounds to all of the lights because I just don't trust screwing the wire to the chassis to complete a ground. I also don't care for suitcase connectors, but that's just from seeing years of problems in marine wiring. Its not any more work to pull a ground wire, you just tape the whole bundle up to the fish tape and pull it all through at once. I also soldered all of my connections and then used the heat shrink that has the glue that melts when it gets hot. It makes the installation bullet proof. The last thing I want is to get pulled over by some over-zealous cop because a side light isn't working. I still need to replace all of my side lights because they were corroded beyond usability, but I left enough slack so that I can rech in and get the bundle to splice into.