Black water tank cleaning

We have had a 192 for 3 years now-I may have a buildup in the black water tank - we follow all the traditional rules of maintaining and flushing tanks and I have researched all cleaning methods  - I turned around the flush out valve which was installed improperly - does anyone have a sure fire method of cleaning out the tank prior to storage? We have tried various commercial cleaners and the 'ice cube' method - 

There are biological agents that can help break down the residue. They do take time. Check out ones like Happy Camper, TankTechsRx, Unique Products RV DigestIT, etc. Don't use anything containing formaldehyde or ones that just mask the odor instead of helping break down the contents. Skip the bleach also. You want to encourage the bacteria that break down the wastes, not kill them. Some people put washing soda (Sodium Carbonate) in the tank. I have not done so. I have been using Happy Camper with our trailer. When I looked down through after draining and flushing with the black tank flush, the tank was almost clean with only a small amount of residue inside. Admittedly, we have only had this one since February and we just completed a month long trip. I had drained the tank while we were on the road, but did not use the black tank flush until we got home, so I think the Happy Camper is doing a good job for us.