So the net of it is that you only have 6V at the fridge? Does it jump to 12V when the engine is running, or not? Do you have proper 12V power everywhere, or is it wrong everywhere?
If the voltage is wrong everywhere, you probably do have the batteries hooked up incorrectly or have one that's bad or something similar. If it's good elsewhere, that oughtn't be the problem unless you have something connected "in the middle" of the batteries in series. There should be nothing connected there electrically other than the two batteries themselves.
I would trace out the 12V power and ground wires from the fridge. One of them pretty much has to be going to the wrong place or have a poor connection somewhere. A good place to start would be to check the voltage of the 12V wire and of the ground wire with respect to a good chassis ground; if there's any noticeable voltage on the ground wire at the fridge, that shows that it doesn't have a good ground connection (though that may not be the sole problem).
