The cost benefit balance is a fine line. Usually a single battery will last two days in the summer, one in the winter. If you don't camp without power, you do not need a second battery at all. Most of us like to have two batteries, that will usually give you three to five days if you are not feeding power hungry children or other needs. It's really worth a lot to have a simple voltmeter on your batteries, and learn about how they discharge over the time you are using them. Helps a lot to know when to recharge, and when you don't need it at all.