I have a sewer gas issue that will not go away. It is particualrly strong on one side of my house... the side of my septic tank. The tank is not full or at least is not acting full... Temp outside has been in the neagtives. Smell is strong when toilets are flushed... seems to go to basement and then into heat vents and spreads throughout the house

Your vent line which gasses escapes must have been punctured, loose or wrongly tapped. Your vent line should extend outside your house ussually at roof. Gas should pass through the vent when you flush, not back out. Check your vent through roof, there could be closed, that's why gasses are escaping inside back. Be careful though. Have a nice day.

One of the likely places for a leak is the wax ring of the toilet, more so if they are loose on the flange. It is a pain pulling the toilet and replacing the ring, so I would carefully check everything else. Make sure you occasionally run water down all the drains. The water in unused traps can evaporate defeating the purpose of the trap.