CLeaning a propane tank residual smell in winter?

I cut mine then built a fire in it

How big is the tank? It would have to get pretty cold to freeze a large tank solid in one week. A simple solution to keep it from freezing would be to toss a heavy tarp over it and fire up a torpedo heater once a day for a couple hours. Insulated blankets would also help. A simple aquarium aeration pump dumping air into the water would also work...just be careful not to use too much soap or it will foam excessively.

Just don't fill the tank completely full and leave
it open on top to expell anything.