voltage drop

[boisesmith]

We have been boon docking for a few weeks and find an overnight voltage drop to about 11.3 volts. This is according to the number of lights on the panel and as shown on my "Nature Power" solar charge controller. The solar panel seems to charge the batteries during the day and running the generator certainly does. I recently installed two new 27 series Interstate batteries. Where do I start to find the voltage loss?