Sadly one of my favorite CG's I like the most has the same problem when the wind is just right and it is SOOO bad you can't breath it's so bad even if you close all the windows in your RV. It's Tom Sawyers CG which is right on the Mississippi and the smell engulfs the entire CG.
AND another sewer smell problem at CG's is from people who dump before they leave and then do NOT put the sewer cap back on.
I am always having to go and put sewer caps back on from these inconsiderate campers because of the sewer smell engulfing my RV.
Put your sewer caps back on before you leave!!