Try the one you have as suggested above. Get a digital voltmeter from your local hardware store and periodically check the voltage on the battery terminals.
13.2 volts is ideal for storage. 12.7 is the minimum. 13.6 is the maximum. The higher the voltage, the more water the batteries will consume. Monitor water level and voltage until you are confident the system is stable. Down to 12.4 volts, means they need a full 14 hour charge.
If the batteries are sealed, 13.6 volts would be a little high. You can't add more water if they vent. Lost water is lost storage capacity.
I wish you good luck and happy trails ahead!