[Carl n Susan]

The best solution for your situation, where you can't depend on the power in the storage facility and don't want to make trips to it to check, is to install AGM batteries (no maintenance, so no water level worries), and as Rames14 wrote, a disconnect switch on the negative terminal.

The batteries will hold their charge all winter assuming they are close to being fully charged when put away.

The disconnect switch stops the parasitic 12V drains. You could just disconnect/connect the negative cable but that gets old. A disconnect switch is easier and not expensive.

The AGMs eliminate checking water levels. Lead acid batteries will work in place of AGMs and shouldn't loose water in storage but still need to be checked once placed back into use.