Unfortunately, we live in the mountains where we are not able to do a lot of winter camping. We would love to, but many campgrounds and BLM areas are usually covered in snow. So we have to save our camping fun for the summer and fall months. In addition, camping in the mountains can be cool or cold even during the warmer times of year so we sometimes wear long johns, hats and other warm clothing items when we go to sleep in the teardrop.
Fortunately, a teardrop sleeping area is so small that usually just body heat will warm the space up in a short amount of time. If you don't like getting into a cold bed there are other ways to stay warm while teardrop camping. If you are lucky enough to have a battery installed in your teardrop, or if you are hooked up to power there are a few heating options. On cooler nights we will use a 12-volt electric car blanket just to warm up the bed. These cost about $30 and don't drain your battery. They will heat up for about 20 minutes before shutting off.
12-volt ceramic heater that warms up the sleeping space. This heater with a fan will drain your battery much faster and you'll notice that any lights you have on will dim while this thing is running. You only need to run it for about 10 minutes to warm up a teardrop bed area. We have also used our friends' propane heater for heat. The Mr. Heater Little Buddy can be placed on a shelf or stable area of a teardrop bed to heat up the space. Don't keep it on all night since a propane heater will deplete the oxygen in a small space very quickly.
Another way to keep warm is with a good, old-fashioned hot water bottle. A teardropping friend of mine has several in her bed: one for her back and one for her feet. She says they are still warm by morning.
One time while camping in 20 degree temps with some fellow teardroppers, we were so cold that we heated up foil-wrapped river rocks in the campfire and then placed them in our beds to warm them up. The hot rocks proceeded to melt my friend's sleeping bag.
This is not a recommended method of staying warm in a teardrop trailer.