Lots of aftermarket battery tender type chargers available which work well for many. I like to just keep it simple and rely on our onboard Iota IQ4 smart converter, with 4 stages it really knows how to keep our batteries very healthy. Our rig is always plugged into shore power while stored in our sideyard, of course I routinely check water levels although they only require watering once a year. Last set of GC2 batteries went 8 years and still had plenty of life in them when we traded them in for another pair that had 30 more amphours.