When we had our tow vehicle wired for lights while being towed, we had the guy hook a line to the tow vehicle's battery for charging. He installed a fuse on the line next to the battery. Our electrical umbilical cord between the coach and tow vehicle is a round plug with seven(?) contact points, one of which has a constant 12v power supply coming from the coach. He ran a charge wire from that umbilical 12v power supply after the plugin on the tow vehicle to the battery. So, yes there is a 12v power supply from the coach... but in our case, it's always charging the tow vehicle battery when plugged into the coach.