Heater is the biggest draw on battery and many would argue that one day is about all you get from your single battery during the winter. Most Class C's come with one 12 volt battery (usually type 24) - you should consider adding a second 12 volt battery (about $70 at Costco) if your dry camping. Not sure how long it takes to recharge using your on-board generator but make sure start the vehicle engine before you try and start the generator ... will put all your batteries on-line to help start the generator and help prevent you from being stranded in the middle of no-where with dead batteries.