If you are camping with access to shore power, a small 1000 watt electric space heater will easily keep the camper warm. However if you are camping off grid, then there are several options. You could use a small propane
space heater (requires cracking open two windows), install a propane furnace
, or just bundle up. When using propane
inside the camper, definitely install a propane detector and CO detector.
EDIT: Many folks use a small propane space heater to warm up the trailer in the evening, then turn it off before going to bed, then turn back on in the morning for awhile. In a recent thread, someone mentioned that most of the propane space heaters produce too much heat. He uses a Martin CH3 propane heater which appears to produce the lowest heat output, about 1500 BTU of heat on the low setting.