If your Tioga is several years old, the seal under the toilet may be bad, or may have been mis-installed. The molded foam rubber donut seal does the same thing a wax ring does under a home toilet, but because it is foam, it takes a set and then sitting on the toilet with 200 lbs, driving down the road, etc, tends to break the seal loose and it vents fumes into the coach.
On person posted that they found theirs installed upside down (when you see it you will realize how bad a problems that is) and another simply said they fixed the fumes by replacing it.
I too experienced fumes while driving east across Nebraska with a stiff tailwind and the roof vent open. I went ahead and changed the toilet seal anyhow. The jury is still out on if it helped, I haven't used the coach but once since the August trip and didn't get any fumes.
The p/n for a Thedford seal is 33239
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Charles