This topic comes up every year, search old threads and you will find a few real solutions and a bunch of unproven ideas.
The proven methods_
_espar heaters, expensive, but since they are made to keep truck drivers alive, they work. Look on eBay and other places for used ones and expect to pay a couple hundred.
_Marine heaters, hold on to your wallet

anything marine costs two or three times what it would otherwise, also many of them require more space to heat safely than a tear will have inside. CT and TTT owners have dones some nice installs. Another item to watch for on Ebay/craigslist.
_camp/deer stand heaters, another idea that requires more space than a normal tear offers, but great for CT and TTT owners. Still not cheap, though more reasonable than marine stuff.
_bunk heaters, reasonably priced, also aimed at truck drivers, according to reports from users here, they will keep a reasonably insulated tear owner warm, and take a few nights to kill a battery.
_gravity heaters, known also as floor furnaces, quite common in TTs until they got too large and required a furnace with a fan. These pop up on eBay every so often and would work great mounted under the tear floor.
_zodi tent heater, eBay is your friend, they stopped making them since they could catch fire if the fan died, the website claims there is a new model coming.
_external heaters, there is a good thread on here where a guy made a propane heater that has a burner outside the tear. The heated air runs through a pipe that runs through one window and out the window on the other side. It works for ice fishing in northern winters, not sure I want to sleep with that going.
_candle box heater, a cookie tin or four stuck to the wall of the tear, since the combustion air is isolated from inside it is a safe design, the jury is out on actual heat gain.
_candle heaters, some claim they work, however you could be breathing the wax that vaporizes above the flame.
_one guy has a tiny wood stove that sits in a box that replaces one door, the stove does not burn all night, it must be reloaded every couple hours...
_unsafe heat, propane unvented heaters, Sterno cans, etc. They consume the oxygen and give off carbon monooxide, it may not kill you right away, but CO builds up in your blood and will give you flu like symptoms, damage your brain and nervous system, and takes many days to get out of your system (you have to grow new red blood cells)
these are all meant to be free of the grid, generators, or inverters. a normal tear can be heated perfectly and safely with a small ceramic heater.
The real issue is insulation, if you don't have it you won't be comfortable. Insulation is good for keeping heat out in warm months, needed for winter camping, and great for controlling noise. With enough insulation two bodies can heat a tear.