For many years in Montana I heated a 21 foot travel trailer with a sheepherder's stove (miniature cookstove -- cast-iron top but the rest was sheet metal). In the way of old trailers, it had thin walls and very little insulation. The stove had a firebox about the size of a large shoebox (IIRC it was 6w x 8h x 16 inches). The pipe went out the wall (replaced a window) and had two right-angle bends, and the cap was about a foot above the roof. Heat circulated around the stove's oven before going up the chimney, so it was fairly efficient for heat transfer (the main flue rarely got really hot). Quite good for cooking and baking.
In mild weather I burned deadwood, construction scrap, even bones and rolled paper (tho that's a pain to keep lit). This was all free salvage. Riverbanks always have lots of standing deadwood. Cottonwood is my favorite as it burns steady, has high heat value for its weight, and leaves almost no ash. You can bank cottonwood down for an overnight fire without making a lot of creosote; other woods will gunk up your chimney unless they're burning fairly hot. Don't burn bark if you can avoid it.
In cold weather I burned coal. This requires a wood fire under it to start it, but once it's started going to coals, it can be banked down for an overnight fire (a banked block the size of your head burns about six hours). The quality of coal heat is much better than wood heat -- the same temperature feels much warmer. The main drawback is that it's extremely dirty, the smoke is to gag you, and about twice a year I had to open up all the littte stove accesses and one of the pipe elbows, and shovel/scrape out the accumulated ash and residue. Coal smoke makes a ton of deposits -- not flammable but really a pain since they grow like fingers on every surface the smoke passes by. On the plus side, strip-mined bituminous coal is cheap; only cost me about $100/year to keep my trailer as warm as I liked. I could easily keep it 80 degrees in there even when it was -40F out. (Once it got below zero, wood couldn't keep up.) Actually the main problem was that it tended to be too warm (and every so often would wake me up by getting more enthused than necessary, but you can throw water on it and slow it down without extinguishing the fire). Burning coal in an open firebox is a black art in more ways than one.
And of course there was the year all I could get locally was crappy lignite (no good for this kind of stove, won't stay lit and has poor heat value) so had to trek all the way down to the mine in Wyoming to get good coal. (Tho it was free for the picking from the side of the road.)
The trailer came with built-in propane heat (a fullsized wall furnace, not the kind they have now) and that was untenable. The propane furnace had to be turned all the way up to keep it halfway warm, and it was very expensive considering how small the space was -- required about 15 gallons per week. And that was with fully adjustable flame, much less costly than they are nowadays with the flame that is only ON or OFF. (For comparison, when I lived in a Real House in the SoCal desert, I once figured out that my wall furnace, at far-cheaper bulk propane rates, cost me $3 every ten MINUTES.)
I did use the flowerpot-on-the-propane-cooktop trick for supplemental heat in mild weather; that uses very little propane and is no more unsafe than cooking with it. But freestanding unvented propane heaters in an enclosed space will kill you.
If I were doing it today, I'd probably use one of those woodstoves the size of a large overnight bag, with a flat top suitable for cooking, and heavy cast-iron sides; Tractor Supply sells 'em for about $300. They can burn wood or coal and the firebox is big enough to take reasonably-sized wood. My neighbor had one of these and used it to heat about twice as much space as I had (but also with no real insulation), and man was it toasty in there. Too big a stove in such a small space and you'll have a lot of trouble with keeping a good fire going without also roasting yourself. Mine was about as big as necessary for coal; could have been a little bigger for wood. (Cost me $20, so no complaints.)
As to the uninsulated metal walls -- as is that's going to be impossible to keep warm. You need to insulate it on the inside any way you can, and outside block the wind as much as possible. Corrugated cardboard and sheet styrofoam on your inside walls are both excellent for the purpose. Old mattresses, blankets, and pillows also work well. Pretty much anything that covers the wall and traps a layer of dead air will work, and you won't spend all your wood heating up the outdoors. If your flue sticks up a foot or so above the roof, and you anchor it with a bit of wire, wind won't be too much of a problem (at least it wasn't for mine, and I lived in a high wind area, 40mph steady with 60mph gusts not uncommon. It was on the downwind side of the trailer, which probably helped.)
Last place I lived in the trailer, I piled straw bales all around it. That's a common trick for folks in old trailers here in Montana -- so long as the straw stays dry, it's excellent insulation. (Wet it still insulates, but it molds.) But if you're burning wood be sure sparks can't hit the straw -- it can smoulder for weeks before it suddenly decides to make flames and go WHOOSH. If you're in a more permanent situation, dirt works great. Doesn't even need to be a thick layer. I think ideally I would put plywood in a lean-to arrangement, and pile the dirt against that -- that way you don't have moisture against the walls, and there's a big dead air space.
Now I'm an old fart and live in a real house just like a real person, but when I was a young'un, I thought my little trailer was the bomb, and loved the idea of turning a shipping container into a house. Do come back and let us know how it goes!
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