Moisture is only a problem in a tight, insulated space. All torpedo (salamander) heaters should have stickers on them that tell you to have x amount of square feet of ventilation to fresh air, that goes for kerosene and propane. I heat my entire house with an 80k BTU gas furnace, but that is fairly tight and insulated. My very well ventilated unattached shack of a garage is tiny by comparison, but gets heated with a 60k BTU heater. It heats up fast, but it also loses heat fast because there is so much "ventilation" and no insulation.
It looks like electric heat isn't an option for you because you just have 120 volt power. Sealed combustion heaters are also out because this is not your property. Tank top, or convection heaters may work if your structure is tight enough and insulated, but if it were me, I would open that garage door about 6" and fire up a used kerosene torpedo heater.