Each sink has an
Air Admittance Valve under the cabinet, near the sink, after the P trap, hereafter referred to as an AAV.
They can fail. They are cheap and an easy screw off screw on replacement. Replace those then run water into each sink trap.
On my RV, the black tank vent through the roof got stopped up. The next time I drained the tank, it killed that AAV. I then had sewer odors till I found the bad AAV. I replaced it and fixed the problem, until I dumped again and killed the new AAV. When I finally fixed the blockage of the main vent, I replaced the AAV again and have had no more problems.
