Sounds like a smoke test is in order to determine where the sewer gases are emitting from. A plumbing supply house will have 3 and 5 minute smoke bombs, use a shop vac for blowing air towards the house from a cleanout outside. Any visible smoke or smell of smoke in the house will lead you to the problem. Most likely could be a leaking wax ring. If you ever pull a toilet and the wax ring is blackened, that is where sewer gases have been travelling between the toilet and ring, not a good air-tight seal.