I did not have any issues using the two stock batteries for my fridge, but I did not boondock. If you are going down the road, the engine keeps the batteries charged. If you are in a campground, the shore power keeps everything running. What type of camping will you be doing that you are worried about losing power?
Also, Interstate and others make batteries that will gain you 50% reserve capacity over the factory units, if you really think you have a problem and don't want to spend the money on adding a bunch of 6v batteries:
http://www.interstatebatteries.com/m/category/rv/rv-deep-cycle
Another option is solar panels. The weight is less and the cost may be around the same.
One more thing that just crossed my mind, the RV has a generator auto-start that will kick in when your batteries get below a set voltage. The auto start is in the "mud room" area to the right of the house door. If you are boondocking, you likely will not have restrictions against a generator.
