PS
Thetford Aqua Chem. It's a liquid, which is far more effective than the tabs. Add a bottle of this (per label) & fill the black holding tank approx. 3/4 & do as dc suggested & take it for a drive to slosh it around a bit, go to a dump station & fill both the grey water & black tank to Full, drain the black tank first, then the grey tank, this will rinse the drop line. I would suggest adding a bit of water (3") to the black tank again along with another dose (per label) of the Aqua Chem & see how it goes. I've used this product when I had my travel trailer (5yrs) & never had an issue. In your case, it almost sounds as though the tank was not treated nor fully drained. PS Don't use bleach, that could cause the seals in the tank valves to dry out & deteriorate. -Mezz