LOVE these black tank posts. :)
RE: the tank sitting a foot below the floor...the tank is sealed. You could set it on your picnic table and it wouldn't smell.
RE: smell when dumping....the sewer is full of grey water, too. Have you ever smelled grey water? It's worse than black.
We've full-timed for 16 years, used only water, and no smells. We've boondocked for up to 12 days at a time....no smells. We run the fan...no smells. We stayed in hot areas....no smells. The seconds it takes to flush...no smells.
All I can say is plain water works for us.