Also, there is no need to get hung up on Group 27 size 12V batteries.
We have a couple of 115 amp hour Group 31 12V deep cycle batteries in our Itasca Class C and they fit right in the stock battery compartment under the step, with room to spare ... for a total of 230 12V deep cycle amp hours of capacity. There is even one Group 31 12V deep cycle battery manufacturer that offers a 125 amp hour version ... a couple of which would provide 250 amp hours of 12V deep cycle capacity.
Wiring a couple of 12V batteries together in a balanced manner so they both charge and discharge at the same internal current flow rates is easy. However, wiring three 12V batteries together in a balanced manner is more complex, but quite doable if one wants to make the effort. I'd like to see how Chinook hooked up three from the factory! The Chinook motorhome we owned only had room for one 12V battery, so it was mostly a CG Hookups Queen.
