There will be more than just the fan running off of the battery i.e., water pump, carbon monoxide & propane detectors, water heater electronic ignition, and lights. Be sure to turn off your TV amp. Don't discharge your battery more than half way. If you do you will reduce the life of it. Always be sure the water is topped off.
2009 Rockwood 2701SS
2008 Tundra Crewmax 5.7