Maybe from sink or toidy?
In the bath, you probably have a vent on the sink trap. These are one way valves, (letting air in to prevent water in P-trap from being drained) but they sometimes fail. You can test by putting a plastic bag and rubber band on to see if odor disappears. If it does, go buy a new one at Lowes, Home Depot, or plumbing supply. They are called cheater vents, are cheap and just screw on.
If your P-trap is working, it shouldn't smell, UNLESS it's bacteria in the P-trap. Running a bunch of water may help.
If not, put a little chlorox in the trap and see if it goes away. It will happen on almost any rig if left to sit too long with bio material in the tank or trap.
A running ceiling vent fan can draw fumes from the toilet when it is flushed.