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Sewer gases can come up thru a drain only if the trap below is not full of water. Using the water regularly is what keeps the trap full of water. If the fixture is not used often enough, the water in the trap can evaporate, allowing sewer gases to flow past it. Often, if there is a problem with the plumbing vent, such as a partial blockage, or the vent is installed incorrectly, the water can actually be siphoned out of the trap, allowing the sewer gas to escape. If the smell is recent and the fixture has been in place for a long time, then it would be safe to say that the vent was installed properly and that something recent may be causing the water to siphon out.
The smell could be coming from the wax seal of a nearby toilet, in which you would just have to remove the toilet and replace with a new gasket. I know that you said you have replaced this, but verify that it was done correctly and that the two toilet bolts are tight, but not overtightened; the toilet should not move.
Loose joints at the fittings are pretty rare, but can happen; unfortunately these are not easy to find without removing the portion of wall where the vent pipe lies behind the wall. Before doing this I would check all visible joints, between the trap and the vent if accessible.
My gut feeling is that the vent is siphoning the trap water. I would watch the short video below and clear the vent as they mention, to see if that does the job.
The first thing to do, is to really try and narrow down where the strongest smell is and where it is coming from. This will help tremendously and once this is established, the things discussed earlier can be used to fix it.
If you need further help or clarification on this answer, please reply to this post.
Best of Luck, Brian
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