First Time Boondocking.....Need a few Questions Answered

Get a heated mattress pad! Use it to preheat the bed half an hour before you get in. Then switch it off, but keep the switch handy in case you get cold at 5am.

This will save you an enormous amount of heating time. Heat the people, not the air.

I use this method, and am toasty and comfortable in freezing nights, without propane.