The propane heater will add a LOT of water to your environment if not a vented one. The only way I know to reduce or eliminate condensation is to either have surrounding materials that will absorb and release slowly the humidity (I had a utility trailer with steel roof and masonite side panels that never condensed. The other way is to eliminate the metal to air interface with spray foam or some sort of coating that won't allow the thermal transfer to allow the water to condense when the dew point is optimal for such. Think tight fitting koozie on your beer can.
I am an sojourner in the earth; hide not Your Commandments from me. Psalm 119:19
Here is the patience of the saints; here are the ones keeping the commandments of YAHWEH, and the faith of Yahshua. Rev. 14:12