dave17352 wrote:
Sounds like a bad seal at the flange. I had one of those toilets with the overflow drain that leaked sewer gas it was awful! My present truck camper appears to have the same overflow but does not leak gas. If it does I will replace it no questions asked. But your if your present toilet has no overflow drain then it has to becoming from the flange or a leak in the vent system someplace. Does your flapper hold water. If it doesn't you will have the same problem.
I have replaced the flange, and the bowl holds water. I have a previous thread that discusses all the possible causes. Now I have discovered that I can make it go away by just plugging the hole where the water comes out. That's where the problem is.