I cannot see the vent pipe coming out of the tank. It must be on top. Anyway, I drained the tank again and now for about a week 5 days everything seems fine. I did check the roof vents again. There are 3 vents and I cleaned all of them again.
I did call Thetford toilets and the tech told me that a deteriorated flange seal might not leak water but could certainly leak fumes. I ordered one and will change it out this weekend. It is 7 years old after all.
After the last episode I stuck a rag behind the toilet and packed it in real good. Maybe that is why I haven't had a smell recently.