Charge em up then disconnect em. No need to leave a charger on. Pay a visit once a month, even once every 2 months, connect up & top up charge. If the batterys a good the recovery will not take long at all.
Farmers & construction companies leave equipment unattended for months at a time. No chargers on them. Same wet cell batteries that RVers use.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter