For my travel trailer, the house battery on my boat, backup power for my ham station, I use 100Ah Sealed Lead Acid (AGM) batteries designed for UPS service. I happen to have a source of these that are removed from commercial service after they get two years old. They typically have a capacity of about 85Ah when I get them. I use them for about three years and discard them when their capacity goes below ~50Ah. To get three years, I constantly float them per the manufacturer's instructions (13.5V to 13.8V, depending on ambient temperature), other wise, just sitting around, they go dead in six months to a year, and cannot be recovered.
Check with local government or businesses that have standby power systems. For critical applications, they routinely remove from service batteries that still have a lot of useful life for a hobbyist.