The concept of leaving one battery discharged to 50% for 3 or 4 days while using the other battery and then it too being discharged to close to the same state, then going home and recharging, is the fly in the ointment. Lead acid batteries are not meant to be stored discharged, not even partially. It's why your alternator charges up the car battery shortly after starting the motor up.
The longevity of a battery is inversely proportional to the length of time it stay in a partial state of charge. The less time it stays partially charged, the longer the longevity of the batteries life, all other things being equal.
Hence charging up every morning, or day, is much better that seeing how many days you can get out of a battery then switching to another battery to do the same thing, and then recharging batteries back at home, when you get around to it. That is an open invitation to sulfating the plates and needs a major PIA equalize charging session with a custom manual battery charger that can take your batteries back up to a full 16 V and a specific gravity on every cell in excess of 1.275 with a hydrometer check. In other words, a whole lot of extra work and baby sitting to get things back close to 100% where they were before you started your camping trip.
Go do a search of DAS26's battery adventures doing what you are doing, and learn from his mistakes.
