New Battery boiling

I did have the motor home hooked up to regular outlet at my home for lights in the rv. But did not have it cranked.

Unfourtnaley no I did not take a picture. I did of the two deep cycle, but not of the cranking battery. But I don't think I got it wrong. Since there were only two to one post and they are connected together and one to the other post. I'll get a picture tomorrow.

Did you take a picture of the cables ?

Replacing batteries seems so simple but it seems like when someone doesn't
take a picture and draw a diagram too they inevitably get mixed up .

Before changing the cables take a picture and post it here .
The folk here are so helpful and they will help you figure it out .

I would not hook the batteries back up until you resolve the issue .
You may have a defective charger that is charging full blast .
At any rate a boiling battery is dangerous .

Tell us what batteries you have and how they are hooked up.

Sure sounds like a mis-wiring! Perhaps something like charging a 6 VDC as if it was a 12 VDC.

were you plugged into shore power?

I would verify the battery hookup. On trailers it's usually black is positive and white is negative. I'm not sure if that holds true for motorhomes.

Do you have a meter that you can use to do a continuity check between the wires you were using on the negative post and a good ground point? you should have almost no resistance with that measurement. If you are reading high resistance or even OL you are checking a positive wire. make sure you are checking the wire to ground, DO NOT put either probe on a battery terminal or you'll blow a fuse or fry the meter.