RVing Drives Me Nuts ---- Electrical Issue

Provided through battery circuit too. Did you try the obvious and clean your trailer connector? Only takes a little corrosion. Normally unplugging/plugging in to the TV will clean off flash corrosion, and if things works again you know some time with a fine wire brush, contact cleaner, and corrosion inhibitor is in your future. I'd start there, then if no joy move onto checking your battery connections and then trace the ground wire to find the frame attachment. If you have a multimeter or test light, you can check for continuity between one of the light sockets to trailer frame, but since it's only one side I'd gamble on being the feed to that side.