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Yes..........you can do that. It is called a "series-parallel" hookup. First hook up the two 6 VDC batteries together in series. That means the positive of one to the negative of the other. That gives you a 12 VDC "unit" with two batteries making that up. Then you parallel that "unit" with the other 12 VDC battery. This means that you take the open positive terminal and hook up the parallel cable to the positive terminal of the 12 VDC battery and likewise with the open negative terminals. Now all you need to do is to hook up the positive leads from the coach to either one of those positive terminals that you just hooked together. Then lastly, you hook up the coach negative(s) to the OTHER battery or unit that you made. Don't hook up both coach leads to the same battery or "unit" that you made from the two 6 VDC batteries. This will give you 12 VDC power into the coach which is what it uses. Hope that is clear. Make up a diagram and put the batteries in the tray in such a way as to make the leads as short as possible. IF you can.............attach a photo of the hookup to your post when finished so I can see how you did it. Hope that proves helpful here. All follow ups are included in our fee.
The link below has some diagrams that might be useful here. Please get back to me if you need more later............http://www.atbatt.com/rv-battery/how-to-wire-6v-batteries-in-series-or-parallel-configuration