Welcome to the world of trailer wiring. Yes you would think red would be hot but as you have found there is a wire that is black that goes to the frame. Guess what the white is hot and the red is ground. Prove it by plugging in your trailer without the wires hooked up. Measure from ground to each wire the white will be +12v I bet and there will be no voltage on the red to ground. The size wire looks no bigger that #8 might even be #10 which is too small to use with two 6 volt battery's in series to join. It is ok for limited use but you really should use standard size like you car or truck has. Where is your inline fuse from the battery? You should have one right at the battery before it goes into the trailer! Do you have a volt meter?
