Great discussion, folks. It's a common problem with most RVs -- not just Thor motor coaches. We've encountered it plenty of times with our cleaning service. Here's a recent article we wrote about this issue and some solutions:
Why Does My RV's Gray Tank Stink?
It's a common problem for all RVs, not just Thor motor coaches. Here's a recent article that goes over some causes as well as solutions. Some of the issues brought up here (vents, seals, traps) are valid, but the likely biggest cause is mold in your gray tank (and DO LEAVE THOSE PULLS CLOSED until you're ready to empty the tanks).
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