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You need to have those batteries connected in parallel first. That is a lead from the positive of one to the positive of the other and the same on the negative leads. Then you have the white negative lead(s) from the trailer attached to one of the negative leads on one battery. The red or black positive lead(s) from the trailer hook up to the positive terminal on the OTHER battery. That balances the system and supplies 12 VDC to the jack and the trailer. Hope that helps here. Please get back to me if this isn't clear as all follow ups are included in our fee.
I am adding a link to a diagram of what I just described. https://tse1.mm.bing.net/th?&id=OIP.M0f9ae223150f2f55fb5ab86a007892cdo0&w=221&h=253&c=0&pid=1.9&rs=0&p=0&r=0
Just checking back in to see if you were able to get this resolved or so you still need some additional input from me here?