Battery switch causing heat up

OK folks let me explain this a little better. I have two 6V batteries in series, so there is only one hot and one ground (red and black) that are used in the system for 12V. The other hot of one battery is connected to the ground of the other battery to complete the series connection. That works fine. Then I added a blue sea switch in the circuit between the two batteries which completely opens the circuit and there is no voltage at the system when open...when closed there is 12V present at the main terminals. I'm inclined to believe I need heavier gauge wire because it works fine if the batteries don't get drained and the generator starts charging them while running the AC. What happened on this trip was the generator ran low on oil and shut off, and at that time the batteries kicked in because the converter was on and they started supplying juice for the converter to put out AC. The air conditioner was on and when the batteries drained down the converter did it's job and shut the system down. So I topped off the generator oil and restarted it and that's when the cables got hot because the generator started recharging the batteries. If I'm not right about all this let me know what's wrong in my thinking what the issue was. Everything is fine now after we got home and I put the charger on the batteries and recharged them. The generator runs fine and the cables don't get hot at all.
Make sense