The odor is caused because the sewer gases are not venting through your roof vent properly. The septic tank should be pumped to ensure that the scum level has not blocked the top of the baffles. The baffles in the septic tank should be inspected at the same time to ensure that they are in good condition. The plumbing system should be checked to ensure that there are no blockages in the piping or the roof vent. 

If these things are checked and these or any other problems fixed, the odor should go away.