Most modern ambulances have dual batteries, but nowadays they don't often have an A/B/OFF selector, just an OFF/ON switch, usually with some kind of electronic load control and a charging management function.

There are some very compact batteries out there, but that seems like an expensive overkill. Something like this "battery saver" may be a better solution. It cuts off the all load from the battery when the voltage drops, saving cranking power for when you need it. All radio presets, etc. are preserved in it's memeory

https://www.amazon.com/PriorityStart-Vol...v+battery+saver