The lowest for me was -11ºF in a Class A RV. We didn't use any heat during the night. I slept with long johns, sweatshirt, warm socks in a 30ºF sleeping bag with another sleeping bag over it. Slept great for the weekend while there. Turned on the propane in the morning, jumped back in the sleeping bag for an hour while everything warmed up. Never really felt cold at all during the nights. Inside thermometer showed low 20's in the morning if I remember correctly.
More recently in a van, Roadtrek 190P, I spent a few nights in the low 5ºF to 10ºF range, covered in snow. Same basic principles, run furnace until bed time then turn it off, long johns, weather, warm socks, but used blankets and comforters instead of sleeping bags. Toasty warm all night. Reached up and turned on the furnace about an hour before getting out of bed to warm the interior. Inside thermometer showed mid 30's in the morning before turning on the furnace.
It's amazing how warm you can be at night in very fold temps if you capture your body heat efficiently. Good layering of clothes and warm blankets/comforters/sleeping bags, and starting with a warm interior at night are key. Then turn on the furnace about an hour before getting out of bed, and the interior will be nice and warm. I also crack a window open to control condensation.
More recently in a van, Roadtrek 190P, I spent a few nights in the low 5ºF to 10ºF range, covered in snow. Same basic principles, run furnace until bed time then turn it off, long johns, weather, warm socks, but used blankets and comforters instead of sleeping bags. Toasty warm all night. Reached up and turned on the furnace about an hour before getting out of bed to warm the interior. Inside thermometer showed mid 30's in the morning before turning on the furnace.
It's amazing how warm you can be at night in very fold temps if you capture your body heat efficiently. Good layering of clothes and warm blankets/comforters/sleeping bags, and starting with a warm interior at night are key. Then turn on the furnace about an hour before getting out of bed, and the interior will be nice and warm. I also crack a window open to control condensation.