Even if you have a battery disconnect the internal leakage will discharge the batteries over time. A small solar maintainer will either slow down or stop the discharge without cooking the battery and without a power cord or controller. I'd use something like this:
15 Watt, 12 Volt Solar Panel
I might even disconnect that and check the battery voltage every week until I had a sense of how long it would sit and plug in the charger when the battery dropped to around 80% charge then plug the charger back in. It's a PITA but some time spent tracking discharge and charge will give you the clues you need to just do as needed. You could do that with or without the solar if you don't mind a cord for a day every month or two. If you live where it freezes it will be more of a problem when it is cold.