DragStuff

I have three batteries in the front of my trailer I just picked up. My issue is two of the batteries are down to 5 volts and one is at 12.7. Each battery is grounded seperately. Battery A (5.7 volts - older battery) is connected to Battery B (12.7 volts - new battery), Battery B also connects to Battery C (5.7 volts - new battery). The positive wire between A and B has 0 ohms resistance and the positive between B and C is 20 ohms.

My assumption is Battery A battery is pulling down Battery C...but I can't figure out why the one is still at 12.7 volts. There is an isolator in the system but it just seperates the recharge wire to the truck from the house 12 volt stuff.

Sounds like you need to disconnect all of them, charge & load test to see which ones are bad.