Safe heater options for small cabin with no electric
There's an absolutely fantastic heater called a wall furnace.  It is a proper furnace that mounts in a wall, and is either one-sided or two-sided.  You could mount a 2-sided unit on an interior joist wall.  Or you could mount a 1-sided unit on an exterior wall by spacing it out from the wall 4", this would let you put a jog in the stack pipe and penetrate the wall instead of the roof.  (I hate roof pens, especially on containers which have irregular roofs). 

A slight variation is the floor furnace which mounts below the floor in a grating.  

They typically are sized for 25,000 to 50,000 BTU and intended to heat an entire, modest sized California or Florida home of 500-1200sf (depending on how well insulated it is). 

They use a millivolt thermostat, which means a thermopile (Peltier device) right on top of the pilot light, creating just enough power to operate the propane solenoid valve.  The pilot must run 24x7.  They come set up for methane, but can be jetted for propane. 

Exhaust is straight up, via convection. Most are not "direct vent" and rely on some draft in the building to fuel them. Keep in mind there is no electricity to force air through the combustion chamber, so the airflow can't be too convoluted. 

You cannot find these in the snowbelt.  This is absolutely daft, because the furnaces they do have in the snowbelt all absolutely need AC power, and AC power fails a lot during the winter, often for days at a time.  I would want one of these at least as a backup.   It would also make a wonderful auxiliary heat for a heat pump system. 

Combined with a modest but carefully designed DC battery/solar system, you could be toasty warm and watching Netflix while your neighbors are futzing with generators.