When you indicate using the batteries to power four sets of lights I would need to know what "four sets of lights" mean - are they grain of wheat bulbs or LED's? In general you would not need all three batteries to power lighting. A 7.2v 4100ah battery would be more than sufficient to power lighting for some time. There are a couple of things you need to consider dependant on the type of lights used:
1. Grain of wheat bulbs generally operate on 6v and whilst they would work on 7.2v it is likely the additional voltage would reduce the life cycle of the bulb.
2. LED's operate on different voltages dependant on the colour - generally between 2.2 to 3.3 volts, a 7.2 volt battery direct to an LED without some form of voltage reduction or resistor will cause almost immediate failure of the LED. See
http://www.componentshop.co.uk/led-exp/ for a very good explanation into LED's
Taking the above into account a single 7.2v battery would power your lights through a quadswitcher.
I will try to put together a wiring diagram to show a method of achieving this, it may take a couple of days to do this so bear with me.
Shaun
