Thanks for pointing out how AGM batteries have low self-discharging rates, no maintenance requirements, and a respectably long lifespan. My husband's looking for an extra battery we can use when we set out on our month-long RV trip along the west coast this summer. I will suggest we purchase an AGM deep-cycle battery because it seems like a very reasonable alternative and option.  https://concordebattery.com/about/lightweight-agm-battery-from-concorde-battery-corporation.html