Having encountered the same problem as yours, I will tell you that everyone here has already given you every possible answer to fix the problem. I tried all of them also, and still failed. My final answer was to call in the Professionals.
See if you have an "R.V. Proctologist" in your area (company name). These guys have specialized HIGH pressure blaster hoses they can shove up into your tanks and they guarantee it will break up any blockages you have. Worked like a charm on mine, and they did all the tanks while they were there. Gave them all a good cleaning, including the hot water heater tank. Yes, it was a little expensive, but I assure you it's worth it in the end when you see what they get out of the tanks!!
Loren
