Keeping he cells full of distilled water, keeping the posts clean and storing the batteries charged up. Seems like you've covered everything that's important.
Since all cells produce 2 volts, a 6 volt battery will have three cells and a 12 volt will have 6 cells. Since both are the same size overall, the cells in the 6 volt are bigger and the lead plates are larger, which allows the battery to tolerate deep cycling (down to 11 volts) much better than a typical 12 volt battery.
I have two totally automatic battery minders to keep two sets of batteries charged. (The two engine batteries and my CPAP battery. My coach's house batteries are charged by the inverter/converter.) Both of these work well and I'd recommend using one. They are both totally electronic and automatic. I believe they are rated at 5 amps. That's enough to keep the batteries charged. Since you'll be charging batteries that have been significantly discharged, you may want to consider a high amperage battery tender.
Good luck.