Battery size and quality
First, look at battery size. I have the factory dual battery setup in cy C25 dinette layout. Neither location will hold a group 27 battery box. They are designed for a group 24. Yes, the 27 is larger, but you would need to buy new battery boxes, then re-build the factory trays to hold them. Battery quality and type. Many inexpensive marine batteries are actually classified as "starting" batteries. They have thinner/lighter plates and smaller capacity. There are more expensive "dual-purpose" marine batteries that can be used as starting batteries and so-so deep cycles. They have heavier plates than starting, but not a true DC. The Deep Cycle battery has heavy plates and is designed to provide a constant supply of power through a deep discharge ans is almost always the most expensive. It can be re-charged then go through the cycle many times. A couple of group 24 Deep Cycles will do the trick providing a reasonable supply of power, especially if you are on shore power and keep them topped up. The more expensive AGM(Absorbed Glass Mat) and Gel batteries are an option and are supposed to have a longer lifespan, although I find standard lead-acid batteries will last me several years with proper charging and maintenance so I can't justify the added $$$$..
