My house has its own sewage system (septic tank) and the house is only 2 years old ( built in 2006). Starting last week, we smell horrible smell from our first floor powder room and one of bathroom on second floor (this two rooms share same piples of the sewage system). All other three bathrooms on the second floor are fine. The bad smell happens only after a shower or bath and it last about several hours. We have used RID-X that is used to treat our septic tank. Unfortunately, it does not get rid of the terrible stinky smell. Does anybody know what went wrong in the sewage system? And any solution for this problem? Thank you very much for your answer my questions.

Does the toilet move? A toilet that moves causes a wax seal to settle and allow sewer gas to bypass the seal.

Again with the plumbing vents.
These stick out of your roof and vent sewer gases up above your home.
If they are semi-clogged, you smell it in your home.
When you take a shower, or bath, these drain large volumes of water which providing enough suction to burp these gases up through the drain.

This is not good.

Of course, the people whom built the home (their sub-contractors) may have forgotten to glue the pipes together, or like my mother's home - drilled holes in the plumbing vents.

It oonly took cutting out the sheetrock, some blue pvc glue and the time to put the sheet rock back.