Sorry about the delay in my followup response. I bought a " aero" roof vent cover. Old one was cracking. On removing the old one I discovered the source of my odd odor problem. The 1 1/2 " pipe was simply sticking through a 2" hole in the roof. No caulking or sealant of any kind aND it wasn't sealed to the vent to cover. So under certain conditions ( nearly full tank) the sewer gas rising up the vent pipe would simply diffuse into the ceiling and walls and out any non-airtight area into the bathroom. After sealing there I haven't had a serious odor problem. I hope this helps anyone fighting a similar problem.