If BOTH battery banks are drained, then the possibility of the House (Coach) batteries have no water(electrolyte) in them. Or are extremely low, which means they are toast and need replaced. The chassis are probably maintenance free, but if discharged too many times will lose the ability to hold a charge. You need to verify that YOUR RV charges BOTH battery banks when plugged in. Not all RV's do, and if so, your dual charging system may be malfunctioning. Another poster stated the mirror heat, THAT should never be a factor as if installed correctly, should be on an Ignition circuit only. There is NO appliance on an RV that will discharge a fully charged battery bank in 24 hours. Even an Inverter will not discharge in 24 hours if left ON. Doug