Dual Battery Hookup on 2010 Elite V C4

[2010 Elite V Oldman WB SS 1]

I just bought a 2010 Centurion Elite V C4.  It has a starter battery and a deep cycle house battery with a selector switch and automatic charging relay.  Currently, all the cables (two positives, two negatives, and a small black wire connected to the negative terminal) are connected to the starter battery.   My question is how do I hook up the house battery. 

My original thought was hook the smaller positive and negative to the house battery but I was not sure which battery to hook the smaller black wire to.  Doing some research, I saw many diagrams and pictures showing a cable connecting the negative terminals of each batteries to each other.  I don't have that cable so I am not sure it is needed for this setup.  Any help would be appreciated.