That said, cars have 12v batteries. If the pi was better (again another gripe, choose better products not just what everyone else uses) you could put the thing to sleep. Even a good low power Linux machine can go years off one of those (quick math says like 100 years, so negligible draw). Or you could cut yourself off if the level gets below a threshold for too long. Plenty of solutions.
I have reverse camera connected to SBC with Allwinner H6. It easily saves around 5 seconds compared to Pi (their closed bootloader takes ages to even start loading kernel). I considered early booting the system when opening the door, but really it is fast enough already.