I had the same issue. I crawled around sniffing, to see where the smells were coming from (no this was not fun). I finally found the fumes were coming from the back of the toilet. I called the toilet manufacturer and walked through a checklist with them (to include checking to make sure the roof vent was not blocked or plugged). After going through it all, the rep told me to take it to the dealer and Thetford would replace the unit. They asked where I would be taking it and they would send the unit to that location, so it was there, when I got to the dealer.
The toilet was there (even though the dealer almost installed it on another unit) and the new toilet works fine. There was a bad seal in the toilet itself.
Thetford did a wonderful job of taking the time to walk me through their checklist and making sure everything was ready when I arrived at the dealer. I was pleasantly surprised.
Ben and Doreen
Home Away From Home - 2017 PT Crusader 315RST
TV - 2008 F250 Lariat 4X4 6.4 Diesel
42 nights camping in 2015
49 nights so far in 2016 (Still Not Enough!)