Inside sewer smell

1. The various air valves are connected to the GRAY system. So, flushing the toilet will not cause those valves to "leak" odors.
2. The most common sewer smell when flushing would be the rubber toilet seal on top the toilet flange.
3. Pour Pine sol in the gray tank and see if you get Pine Sol smell when flushing. This will narrow the cause down
4. IF you have a Clothes washer, make sure you have water in the washer P trap. Especially if you rarely use the Washer.
5. IF you are using Black Tank chemical(You SHOULD), then there should be no real smell coming from the Black tank.
6. Last, but I have seen this in Tiffins also, the 3 inch down pipe attached to the top of the Black tank from the toilet may have come loose or is NOT sealed and that will cause Black tank odor into the RV when flushing. The only way to check this is to pull the toilet, but if you need to check the toilet seal(item 2 above), then it is not a wasted effort. Doug

Don't buy the cheapest one you can find. Spend a little more and get a good one.

You could also empty your tank and give it a good flushing. Perhaps put in some toilet chemicals.

Air Admittance Valve - Amazon

Commonly called "studor vents" by the commercial plumbers around here.

My wife calls them foo-foo valves...lol

Its called an Air Admittance Valve among other names. I would imagine that most RV or plumbing supply stores would have them.

Some screw on, others just slide on, so if you change yours, you might want to remove yours first.

Air admittance valve.

You can purchase them at Lowes or Home Depot. They're about $8.00 each. They have them with a male and female thread so get the correct ones.