If my furnace were unsafe there is no WAY I would ever sleep in the trailer. It's ONE thing to have a problem while I'm somewhere else and something FAR more serious to have the same problem while I'm sleeping. If in doubt have the furnace inspected so you know it's sound then leave it run as much as you like. Since you now say there is no electricity you will get very little time from a single battery running the furnace. What happens when it no longer spins fast enough to have sufficient air flow? It simply shuts down.