I put my batteries under the seats in the living area along with the inverter. I have nine batteries under there and room for plenty more if I needed. You can see some pictures at the article I have been working on, feel free to add/change the information and add any pictures that can contribute.
I have six AGM batteries in a bank, three deep cycle lead acid (which I will not replace if they go bad), and my engine battery is an AGM battery (YellowTop Optima). I won't even mess with lead acid in any of my cars anymore, they have done too much damage and made huge messes in the past.