Hi,
You should charge them as a 12 volt battery. This will give equal amperage into each battery. However you could charge them separately as 6 volts on that setting if you wanted to fill each one separately.
I have power at my RV storage lot, and keep my RV plugged in. Otherwise if not for the covered storage, my solar would keep it full.
If you have uncovered storage, consider a 100 watt solar panel, it will keep it full all the time.
SunElec.com has some 80 - 120 watt panels for less than $150.
Fred.