Most likely it is not "Sewer Gas" from your black tank; but rotted food and grease in your gray tank.
A good cleaning with a bio-enzyme digester like RV-Digest-It or Rid-X septic system treatment will sweeten that tank. Use warm (not hot) water to fill the tank after dumping 1/2 bottle of RV-Digest-It Unique RV Digest It Holding Tank Treatment Cleans Sensors Unclogs Black Gray | eBay
into the tank. Let stand a minimum of 72 hours (a week is better). Then dump and flush with fresh water. Repeat if REALLY dirty.
Also, traffic movement can slosh enough water out of your traps to leave an air space so that the smell can get past the water filled sink/shower trap.
Refill the traps by running water into them when you get where you are going.
Lou and Laura with Bella - German Short Hair Pointer
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