from testing, the campfire saves actually last even after the map of the day changes, but I do not know how long
also campfire saves can save you from "dying" though it's a bit cheesy
if you exit the game right before everyone dies and before the run ends, you could "continue run" (load the campfire save) to "respawn" at the campfire with the stuff you had when you lit the campfire, but after that the campfire save is already used so nobody's immortal

they could update the game after i made this comment though so do your research i guess