I have only one 12 volt as I wasn't saavy enough to negotiate a second one.
I had 4X 6 volt batteries in my Wildcat and it was awsome. I talked to the owner of our Batteries Plus store (I think he would know :P) about adding 2X6 volt batteries to my new 12 volt battery. He said you can't do it because they are different. My mind wants me to believe 12 volts is 12 volts but I took his advise and am running the single battery for now. I do a lot of boondocking and it doesn't keep the charge long.
I have not measured yet but with the pre-wiring/enclosure for the Onan genset, the battery compartment looks pretty small and I am nervous that 4 batteries may not fit. Has anyone put 4 batteries in the compartment (drivers side) with a generator enclosure installed?