One way to help slow down freezing propane tanks is to have them plumbed to work two tanks in tandem for one burner. I have used this system for
over ten years for firing raku at workshops and on my one kilns. Also make sure you have a regulator on the tank and a burner with the right BTU output.
Otherwise you could either wait for days trying hit hit temperature or you could send too much propane out the chimney.
For a soft brick well insulated kiln you need about 30,000 btu's /hour per cubic foot at ^10.