The tub is not often used. We tried bleach and later, baking soda. Nothing changed.

Are you able to get to the tub trap? Water evaporates over time and will open trap to gasses. If you add water to trap and you still get smell, your trap may be damaged and need to be replaced or repaired.

Chuck

I agree with Chuck that it is probably a dry trap since you say it is not used very much. If there is a problem with the vent, the water might siphon out of the trap as it goes down the drain. Also, vent problems would probably cause gurgling during draining after using the tub.

So here is my (non-professional plumber) suggestion:
Pour about 2 cups of water very, very slowly down the drain. My theory is that by going slow the water won't siphon and the trap will remain full until it evaporates.

At least it's worth a shot. I'm open to correction by someone more knowledgeable in the field.
WallyH