WOW! Great site and helpful Q&As. I, too, am considering a small (30,000btu) propane fireplace for temporary use in a 3200 c/f space here in S. Arizona. The fireplace recommends a minimum 100 lb tank, but with no vehicle to transport it vertically for refilling and no vendor interested in filling such a "small" tank.
Understanding how propane reacts in a tank when cold and what pressures are necessary, I have decided to go with a 40 lb. tank located outside through the wall, 24 inches from the interior fireplace, and insulate the tank within a simple, breathable enclosure, possibly warmed within with a 40 watt light bulb on the "coldest" nights (30-35 degrees).
With a two-stage regulator in place, I can see no problems with keeping the tank wall cozy and expect good results, especially when use will be minimal for short periods of time. The room is well insulated (adobe block), a ceiling fan, and the vent-free fireplace has a thermostat.
Do any of you foresee any downsides to my smaller tank concept? With thanks. Jeff