I have that Zamp plug on my Venture Sonic. You will measure +12V plus on the connector when connected correctly to your two 6 volts in series. + is the female pin and - is the male pin.
I used that male two pin plug to provide +12V power for a handheld spotlight. Got the mating 2 pin female plug from Waytek Wire. Part# 37110. Then got Part # 11015, soldered the two together and now have a power port plug to plug in any +12V device with a standard cigarette plug on it. Be aware that Waytek has a $25.00 minimum order policy.
When I go with Solar, I will not use that plug for it's intended purpose. My wiring will be portable Solar panels to a controller mounted near the batteries, then controller to the batteries. The voltage drop of even 10 ga wire run from the back of the trailer to the front will be enough to cause the batteries to not fully charge. That plug should have been at the front of the trailer!