09 chevy express 4500 van/ ambulance: Over the past Week I've been watching one of the 2 batteries overheat while the truck is off or running. Ive since turned the power inverter off And have been testing the batteries. Both batteries show good on a load tester and work as they should. The onboard inverter says they're showing low and charging in the low charge setting. off charge voltage is 14.1, on charge voltage is 14.2. Weird experience: I sat my tester electrodes down on the good battery(totally disconnected) after getting readings and the meter shows 5.6volts. what would cause this? I'm trying to diagnos the batteries. Any ideas. New readings totally disconnected batteries 12.83 boiling hot and 13.17 cool. I'm up for changing the batteries but should i look for any other issues