You do need a working auxiliary battery. There are a few things that work off of that. Most refrig need it to start up, all lights when not connected to electricity, the furnace at all times. Wether you need on the way home is up to you, but I think the seller should replace it so that you can fully check out the camper. For instance, does the furnace work, or refrig. If there is a slideout, that may or may not need the battery.
I would suggest you hire a RV mechanic to check the unit out if you decide to buy it. From you question, I take it that you are a newby to RVs.
