Have you tried contacting Northstar and asking them? I had an Optima AGM go bad in the first two years and they replaced it for free even though it was a week out of warranty - no questions asked.
Question: are you using AGM for both batteries? Or only for the inside? You can't run an AGM and a regular battery together - if you're doing so, that very well could be your problem. If this is the case, by 2 AGMs and install them both.
If not (had to ask), then how are you charging now? Are you staying at hookup sites? If so, then just run with one battery. Get a cheap regular battery as a back up in case your last AGM dies before you get home.
If you are not staying at hookup sites, how are you charging the batteries? Are you using a generator? Are you using solar? Are you only charging when you run the vehicle?
How long a battery will last will depend on several factors: how much you're using the laptop (all day every day, movie every night, or just 10-15 minutes twice a day), how often you run your water pump, and how much battery power your fridge needs to run its computer components. I would say that with a group 27 or higher battery, you should be able to go at least a day without having to charge if your laptop usage is not extreme.
