I'm 100 percent certain this is not a water-based smell. We had that at one time -- that is, the sulphur smell -- so I would recognize it again.
As soon as I dump, the smell permeates the entire living space. On initial sniff it always hits us as a black tank smell, but I accept the proposition the it could be funky gray. To that end, I have been doing the GEO treatment in all three tanks, and adding a commercial anti-odor product as well (cant recall the brand at the moment).
But thanks for helping me think this through.
2019 K-2500 Duramax Crew Cab
2014 Silverback 33RL