Travel Trailer Electrical

Hey all,

I've been trying to find the causes of weird electrical issues in my new trailer. It has a 160w solar panel, solar controller and 2000w inverter from the same manufacture. Also the converter, and generator. It has 2 6v batteries and uses either 240 or 120 shore power. The main symptom is the batteries barely last through the night even with very few loads. Lights on it'll measure 3-4 amps and the batteries are rated at 220ah each. Here's where it gets interesting. I was moving a 12v light switch and needed to find + - on two wires. Testing behind the switch panel I found 20v on a couple terminals. This was at night and the trailer unplugged from shore power. I checked voltage at batteries and found 12.8 volts. A little while later the same terminals resulted in at battery voltage of 12.6. Any ideas what could produce an irregularity in voltage?