I'm with Jeff, it seems like someone has rewired your rig and you now have 6vdc supplying your trailer. My question is do all the DC circuits in your trailer work?
It looks to me like the battery cables were home made. Look at "The 12 volt side of life" website. It will give you all the information you need to set things up properly. I suggest this site because it is what everyone will tell you anyway and because for me having the drawing in hand is easier than having this thread printed out and in hand. Everything is on one page.
Good luck and try using some quality cables from NAPA or high end part store or automotive electrical shop.