Battery switch causing heat up

Jerry Amerson wrote:
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.

We need to get your terminology straight here......because it is important in understanding what is and is not happening.

First, if you used the same gauge wire(s) as was originally there when you inserted the switch, hopefully there wouldn't be a problem IF you made really good connections.

BUT THEN.....the "converter" does NOT produce AC from DC. The converter/charger goes the other way; it produces 12 V to charge the batteries and other things and is ONLY active when there is an AC input from an external source, like shore power or a generator.
When the external AC input goes dead, so does the converter.

What you are trying to describe is an INverter, which takes a DC input and produces 120 V AC output.

While it is not impossible to have an INverter big enough to run your air conditioner, it is not usually practical because of the high power requirement of the A/C. It would require a large, expensive INverter and would have a HUGE current draw from the batteries. Two would probably not be enough.

So.....is your RV really equipped to keep the air conditioner running when the external AC power fails ?? I HIGHLY doubt it.