I have propane heaters and for the past few days it has been smelling very bad.?

I have had the gas company come in and tell me that I have something in the air that is making the odor, but I have not changed or added fragrence in my home for the past month. The gas company chnaged propane tanks to see if it was bad gas but it still smelled very bad. I allowed entire house to air out by turning gas off and open all the windows and doors several times through this proccess. The gas company's final step was to put a new gas unit in and see how that burns. The odor is not as bad but it is still there and has been there for a few days..... WHAT DO I DO?