Hi Ron!
Sorry for the late reply, festival camping and the holidays have had me 'all-out'.
There IS a big empty space where your batteries are located.

But I was hoping to fit something else there.
Forward of the side door is a larger empty volume that I am looking at maybe mounting batteries. It would be a great place for them... low down and much closer to the front. I happen to have a spare 2000w inverter, so 'house inverter' is not out of the question. We plan on ripping out the carpet anyway, so things will be MUCH easier to see then.
We 'boondocked' for the festival (5 days) and just running the 'frige we had to start the generator every day to charge the battery. It lasts almost 3 days until too weak to start the generator.

So we ran the generator an hour a day as insurance. The coach battery is a newer (2016) group 31 truck battery, but NOT a deep cycle. Unfortunately the compartment for the group 31 is in is VERY tight to try to fit (2) 6V golf cart batteries. (plus the compartment is not vented... thought that was a must..) I may try anyway. Probably just get a deep cycle group 31 for now.
BUT... to try to get back on topic..

I would consider the twin 6V deep cycle a MUST HAVE on the RV.