Several years ago I had the same problem. Took it to an RV repair shop, they couldn't figure it out after replacing the plugs on the TV and TT. As usual I found the problem. The ground wire for the rear lights disappeared behind a small plate screwed to the rear bumper. I took the plate off and both the bumper and the plate were painted. The ground wire had about 1/2" of the insulation stripped off and was squeezed between the painted parts. This some how had worked for several years??? I put on a crimp on, on the wire and attached it with a sheet metal screw. Problem fixed. Always suspect a bad ground!!
Retired and loving it.
Got a new 2014 Grayhawk 31DS
Hoping to get away from the IL winters