On my '58 Traveler I found the cord from the TV comes into the trailer at the front from the left or street side. I cut a small access hole in the inner skin (maybe 2" square) and then cut a larger hole in the floor and used a small deck plate to close that area.
The rear light wiring comes up to the left first and then over to the license and right side. What I did was to make notes & then pull all of the lights. I then wired the front of the trailer to the standard wiring;
After I got the front wired I was able to go to the rear with a meter. I connected the TV and double checked all of the connections.
You should be able to prove the ground wire with a meter but on mine the black was the negative. PS I wouldn't use those plugs just because of possible corrosion, I make sure I use a T&B Crimper and if concerned about moisture then seal the connectors with liquid tape or similar.