I'll add to that, get a dual battery switch. Mine, which is out of an old ambulance but is still available, has off, battery 1, battery 2, both. So, two 6V in series with positive going to leg one, two more 6V in series with positive going to leg 2.
You can just turn them off, or be on battery set one while you run an independent charger on set two, or vice versa. Or tie them together to charge but it will take a typical house converter a good while to top them up.
Plus, when it's time to go home you can turn on one set and not overtax your vehicle charging system.
My switch is a Cole-Hersee Marine switch. Here's the modern version of what I have, you can go much lighter though since this isn't likely to see starting loads.
Battery-Related Products | Battery Selector/Master Disconnect SwitchesM-750 | Cole Hersee - Littelfuse