For those whose RV is connected to a drain most of the time, you probably keep your gray water valve open. At times a smelly little problem arises: Sewer gases can come back up the sewer hose, into your gray tank, and slow migrate their way up the through the tank and out the roof vent. At times those same evil essences can somehow re-infiltrate your RV with a negative impact on your olfactory organs.Worse still, if there are any "vague" connections in your gray water venting system, those nasty little essences can also make their creeping way out, carrying not just the odor of gray water (as bad as that is) but with a direct connection to the city sewer (or local septic system) that REALLY horrific scent of black water can invade your premises.
What to do? Enter the Folgers P-Trap solution: If you leave your sewer hose connected and the dump valve open, stick a P-trap between the sewer and you. Simply obtain a coffee can (or a large rock, and make a bend in your sewer hose, as illustrated in the photo here. Gray water will advance over the bend, but the water trapped in the hose on the "rig side" of the coffee can will prevent the odoriferous fumes from crossing over.
Mind you, when you dump your black water you'll want to temporarily remove the coffee can to allow a free-flow of black water, etal, to make the trip down the sewer unimpeded.