creativeclover said: I have a rather large propane tank. Not sure if it would be good for it. What do you think?
You'll certainly need drainage holes! If the holes are large, and in good contact with soil, worms will be able to find your composting stuff and help out. You need to keep it moist, but if water accumulates in the bottom, your wonderful aerobic composting microbes will die and be replaced by nasty, stinking, toxic anaerobic fermenting bacteria. Those bums will pump out harmful alcohols, organic acids, ketones and aldehydes. The water HAS to drain out so air can get in.
Compost piles also need good aeration, perhaps many large holes along the side of the tank. Hopefully people who have used bins and barrels for a while will chime in about what is "sufficient" aeration. I only compost in an open, shallow heap, so I don't know how many holes you need when you seal a compost heap inside a metal shell and still have it stay 100% aerobic.
Another issue: consider how much work it will be to shovel plant matter in, shovel compost out, and turn the heap. You have to lift stuff up and over the lip to put it in. You have to reach a shovel down into it to scoop any out. (Hopefully you can dump the tank empty when it's finished.) How do you turn compost inside a tank?
If it were just a heap sitting on the soil, that effort is minimal. Dump a wheelbarrow, stir with a garden fork, hoe or rake the finished compost into a wheelbarrow with its nose tipped down and wiggled into the pile.
Really, though, what do I know about that way of composting? I haven't tried it yet.