Many heaters like that have a DSI (direct spark ignition) that only lights when thermostat tells it to run. If it doesn't get propane to fire in a few seconds it faults and shuts down. To get it going again shut it off then back on again and the fault will clear. Just do this once or twice and propane should reach the burner and light. The DSI should be audible while it attempts lighting and ignition also a noticeable sound.
Propane should last couple days per 20lbs bottle in cool temps but the problem will be the blower fan. It sucks back a lot if power and if on battery 12v it won't last as long as the propane. Also, and make special note of this, it has been my finding that this deficient battery event will almost certainly coincide with the coldest night at the darkest hour and likely in the rain while you are at the peak of need for sleep. I'm sure the popup engineers designed it this way laughing to themselves as they drifted off to sleep in their warm beds hugging their soft pillows goodnight. There are many solutions to this predicament but none that are consoling at the time. Just remember: it's all part of the fun.