Check the air admittance valve under the sink in the bathroom. Should be a vertical pipe coming off the sink plumbing with a valve at the top. Most RVs have super cheap valves that are known to fail. When the valve fails, it allows air from the grey tank to come into the RV.
Replace the valves with a high quality valve. They are available at any home store and a good valve should be in the $25 range. Avoid another $4 one like the ones installed on your camper now.
I just replaced mine with those. Screws right on, no tools required.
If you open the door to your cabinet under the sink and get a strong sewer smell, then the vent is your culprit. You can test it by placing a bag over the vent and seeing if the odor goes away.