Nothing wrong with using 6 batteries in your system. The less you draw the batteries down, the longer they will last.
I have six batteries in 3 separate battery banks on my RV with 400 watts of solar panels for a total of about 650 amp-hours. We use quite a bit of the microwave and lots of TV at night (TV and DVD player), heater runs all night even in the summer, and we use about 15% of our battery capacity according to our trimetric. The worst we saw was about 50% usage because someone forgot to put the fridge on propane only.
Will be putting on another 300 watts of panels next year so we can have faster charging to 100%.
PS: We camp in the Colorado mountains. Temps got down to the low 40's at night and it was a welcome relief compared to the 90 temps in the city.