I've been where you're at. After having the tanks power washed we learned of our mistakes. Now, when it is time to dump the black tank, wife stands over the toilet with the trap open and I fill the tank from the outside with the tank rinser. When she sees water coming up from the bottom she knocks on the wall and I pull the handle. Woooosh! Repeat and then close up with 10 gal of fresh water and your done. Always flush a lot of water and bingo. No more problems. Geo method if you're so inclined and you should not have any more problems - we haven't.