I've had this problem with my brakes on and off for several years. I checked all of the grounds, all of the connections and the wiring on both my TV and trailer with no success. In desperation, I decided to bring it to a shop that does truck and trailer repairs. To make a long story short, he had the TV and the trailer in the shop on several occasions, replaced the brake controller and the receptacle at the rear bumper, cleaned the terminals on the battery, and replaced some of the wiring under the dash. He said everything was fine and when I took it out again, same problem.
Last spring, just as I was about to put the trailer away for the winter, I was driving over some bumps in the road and noticed that just as the trailer axles passed over the bumps, the brakes started to work, then went out. I said, there has to be a problem with the wiring at the axle, so last week, getting the trailer ready for this year's camping, I rolled it up on blocks to give me room to get under it (those older Sunlines sit real low you know) and with the brake pedal depressed, I shook the wiring. That's when I heard the brake buzzing stop and start on the front axle. I went to my local NAPA store and picked up some two lead trailer wiring:
and used the old wire to pull the new one through the holes in the axle, siliconing the entry and exit points to keep the wire from wearing through on the steel edges of the holes.
Once I had the old wire out of the axle, I could see the problem:
That's the wire that was about mid-way inside the axle tube.
Now, I'm not so sure it was a good idea to pull the new wire through the axle tube like the factory did, because apparently any slack in the wire will cause a chafing against the inside of the axle tube. As it rusts, it probably becomes more of a hazard to the insulation on the wire. Of course, the wire I've replaced it with is now double insulated, so it will be years before that ever becomes a problem, but I think I'll wire tie the wire to the other axle when I do that one.
Hopefully, this might save some headaches for anyone with a similar problem on an older trailer.