I forgot to add I replaced my original 12-volt dual-purpose batteries with "real" deep-cycle 6-volt batteries. Mine are Trojan T-105 units (
T-105 Plus | Trojan Battery Company) and I equipped them with Trojan's HydroLink watering system (
HydroLink Watering System | Trojan Battery Company) which makes battery filling a 5-minute, no mess, foolproof way of ensuring proper water level in all cells.
I believe Trojans's T-145 batteries will also fit. Slightly more capacity, slightly taller, more expensive, and not as widely available (here at least).