If you have the 15,000 btu AC, it also is a heat pump. Heat pumps run like an AC in reverse, taking heat from the outside air and bringing it inside. The air is delivered through the ceiling ducts. It will be warm, but not hot like the furnace. The AC/heat pump needs shore power or generator to operate.
The furnace runs on propane and battery power. It delivers air that is very warm (even hot) through floor vents.
The heat pump, if connected to shore power, is heat you are paying nothing for, where the furnace is burning your propane. The heat pump is especially effective at taking the chill off on a cold morning, not so effective when outside temps are below freezing. If the heat pump can't keep up, you will have to switch it off and change to furnace. There is no way for it to automatically call for heat from the furnace.
2012 Forest River Sunseeker 2300 Chevy