Originally Posted by TGR
It is the normal trailer wiring convention to have the black wire positive and the white wire negative for the battery connections with the negative circuit tied to ground.
On my 21DK, FR used red for positive and white for negative on DC circuits. White is grounded at the frame and red goes to the circuit breaker under the front of the trailer.
That's true at the battery. Internally they use two-conductor wire that looks like lamp cord but probably is heavier gauge. It's white with a red (or other color) stripe, the stripe side going to the fuse and the white going to the common ground block.