The Airstream has dual electrical systems, 12 VDC and 120 VAC. On my '72 all of the lights are
12V. All of the venilation fans are
12V. The furnace blower is
12V, as well as the on board water pump. My 'fridge was replaced by previous owners, and is a 120 VAC model. You may have a propane/12 VDC /120 VAC model, or some variation. My TV and microwave oven are 120 VAC only.
The battery is used to power the
12 volt side of things when you are not connected to "shore power" as we like to call it, when you are in transit, or if you are camping at a site without 120 VAC power.