I finally realized my keg fittings were infected. Now I disassemble them and BOIL fittings and lids. Lye and Iodophor for the rest of the keg, agitated with a one-use cotton rag on a stick, turned by an electric drill.
If you have organic matter in a vessel you're having trouble removing, buy some lye crystals in the drain cleaner section and put some in the vessel with hot water. It will dissolve anything organic (works on pesky corpses, as well).