Having another 12 volt battery in case one dies is called "redundancy".
It's always best to have a backup to as many of your systems as possible when out and about in your RV. I hate having a trip ruined, or made uncomfortable for the remainder, due to equipment failure.
i.e. Last year our traveling friends had their RV furnace fail hundreds of miles from home in a beautiful area, so they called the trip quits and went home, leaving us to remain in the area for several more days to continue exploring. We were drycamping in boondocking country so it would have been possible to run our generators nights - or at least much of the time nights - powering electric heaters to maintain some warmth in our RVs for sleeping. (We even have two generators with us at all times.)
