I'm finding that these nasty odors can be in fact the tile selection itself and not the drain or plumbing line. On my own home we had to remove the sheets holding the mosaic tile together.
The tile was made in India and the glue used to hold it to the sheets smells aweful when wet. I suspect that this smell is coming from the glue used to hold the tiles together and the smell leaking through the tile and grout.
If you have a spare tile or sheet of tiles try soaking them and then smelling the back side of the tile. If the odor is bad you might have found your source of the problem.
JW