Overnight battery drain

If you've only charged the batteries at intervals of two months to six moths apart and haven't used a disconnect switch to separate the batteries from the parasitic draws, your batteries are probably toast. In between charges, the batteries are drawn down to nothing. There aren't too many FLA batteries that will endure more than a few of those cycles.

Your easiest, hands off option is to install some solar. You won't have to disconnect or connect anything, the solar will provide some power and charging during the day when in use, and will allow your batteries to become fully charged when stored.