At my wit's end. Septic smell emanates from one bathroom and into room attached, but not from any other wall opposite of bathroom, and in no other room/bathroom. Cleared bath drain (which drained slowly), pulled out some hair, now every drain runs fine, still stinks. 2 Roebic treatments, some muriatic acid, still stinks. Augered vent pipe, ran water down for 5 min, no effect. Smell disappears right after shower/bath, but comes back during night. Plumber pulled toilet, ran camera through the line, didn't find any leaks, replaced wax ring, $200 down the drain (pun intended), still stinks. No water seapage or signs of leaks on opposite side of walls from bathroom. Poured 2 pans of boiling water down drain with Roebic one night, didn't smell for 1 day, then stink came back. Attic smells fine.

Not sure if it matters, but had a slab leak a year ago and ran supplemental line from adjacent bathroom to stinky one.

We have a septic system, can't run gallons of bleach to "kill" smell.

Not interested in making our plumber's next boat payment, but I am desperate for some professional advice. Thanks in advance!

Does smell emanate from tub drain? If so, slowly pour about a quart of water down tub drain. See if smell dissipates. If it does then flush toilet and see if smell returns.

First answer Bobs questions.
We have a septic system, can't run gallons of bleach to "kill" smell. No, but what you could trey is tom pour a quart of b leach down the drain and let it set un the trap all night. In the morning flush it out with a pan of boiling water. Let me know if that helps. Good luck, Tom

Might just be a $3.15 wax ring under the toilet. Worth exploring.

It might be the p-trap of the bath that is crack. It could be also a vent in the wall that is broken.
In which matériel the drain is made of...