After a few months of "engineering" and scheming, we have finally completed adding two batteries to our coach battery bank, giving us four batteries with a total of 368 AH for boondocking...
We first replaced the original OEM flooded batteries with two (2) AGM 12v 92AH batteries. We then added two more of the same in the cabinet under the kitchen sink.
We ran 2/0 cables to tie the two new batteries into the existing bank under the steps. This made for a four foot cable run down through the floor and into the existing battery box.
After making sure that we weren't going to drill into one of the STEEL TUBES supporting the floor (and getting up the courage to drill 1 inch holes in the floor of our coach) the actual install went pretty smoothly...
And the only thing we lost was room for our waste basket that we kept under the sink. More than a fair trade in my humble opinion....