Originally Posted by KNMIB
...I am curious, as I watch it snow and freeze outside if the furnace works with just the house battery power. ..
Built-in RV furnaces are a 12v DC appliance. It will work if the only power you have is the house battery, (typically, about 8-10 hours of furnace use in an insulated motorhome for one 12v deep cell).
When you have hookups your power converter is charging your house battery, (supplementing it), and will simultaneously supply 12v power to all the doodads than use it, (lights, vent fans, tank heaters, water pump, inverter, and furnace).