Originally Posted by Kepple Services
Its easy to do, I did it years ago on a dump trailer I had. I ran a 4Ga wire off the battery, with a 150amp fuse, to a heavy duty 2 conductor plug and a corresponding plug off the batteru terminals on the trailer battery. I also got a better ground on the trailer doing this. Its a plug similar to the ones used on tow trucks.
I used one similar to this one.
What he said, you may also want to check the condition of the trailer battery, 5+ years old and its most likely shot.