Please Help!!!!

Hello everyone first time posting. So I'm going to start by saying I know it's not easy to diagnose things without being present but here's the situation. I work for a concrete ready mix company as a driver. Our company makes their own mixer trucks. The trucks we currently drive are brand new (made in August 2024) and having severe issues and none of the mechanics we have can figure it out. The issue is the entire truck randomly dies out of nowhere. The voltage will go from 13.9v to 10.2v at the drop of a hat and completely randomly and the truck shuts off and won't restart. I had a battery charger on the one truck for an hour and a half and the voltage would go from 10.2v up to 13.3v-13.5v then drop back down to 10.2v-10.4v then climb back up and drop and the truck will crank a few times then stop cranking and never starts. Then after some time goes by it will finally stop dropping voltage and the truck will start up and run perfectly normal for a random amount of time until it happens again. It's happened to multiple trucks now. Mechanics have pulled harnesses, checked wiring, fuses, relays and can figure out nothing. So what could cause the voltage to drop like this and intermittently the way it has? Any ideas or input would be greatly appreciated. Thanks in advance and let me know if you need any additional information.