We have propane appliances and heat. Our furnace and oven are filled with black soot. What would cause this? My daughter mentioned letting the propane get low may be the cause. We did let it get low once about three fills ago. Can someone assist with this issue?

If your description didn't miss anything that we should know, I would suspect that your gas pressure is incorrect. That will require a professional to correct. That is, unless you have a manometer and KNOW how to use it and what you are doing. Get that checked soon. It could be dangerous.

Additionally, the available combustion air in the structure may be insufficient. If the furnace is truly sooted, it should be cleaned.