Actually, I am quite happy with one battery at a time! I do not hook them up in parallel or series or anything. One is just a spare. I monitor the voltage level on the one I am using. When it gets to 12.1 (after three or four days), I switch them out. If we are boondocking for a week or so, I then use the generator to recharge the one that needs it.
By not hooking them up together, I have some protection against a sudden and complete failure. If they were together, and one were to die, I might not know it and then be left with one good but depleted battery.
I know this is an unsophisticated solution, but it works for me.
