Wow, thats the first I have heard of that one. The fireplace affects the sewer gas odor. Hmm. It may take a little more diagnostics. My only thoughts are how the plumbing system works. There is no connection physically between the two systems (well there should not be). The connection may only be coincidental. The sewer gas, most likely, is there all the time. Septic tank systems are affected by wind, by water volume, and by clogged venting. For any one of the aboved mentioned items sewer gas can be expelled into the home. When the wind blows across the roof of your home, a certain amount is diverted down the vent and creates a high pressure area inside your plumbing system. If the pressure becomes greater than the ability of the trap to contain it, it bubbles up into the room carrying with it the sewer smells. If your vent is clogged, when you put one gallon of water into the waste system, it cannot go in without first pushing the air ahead of it down the drain. This again creates a high pressure area ahead of the water. If there are no vents available to divert the pressure up to the roof, it will bubble out the traps. One gallon of water must displace one gallon of air.
You will probably need to observe this a little more, if its really a problem get a professional plumber out to check it out.