I had this exact problem years ago with one of my Big Dogs. Tried all the stuff you have, then one day I was polishing the billet aluminum on it and thought about all that nasty black residue on the rag. Then I took a bowl with hot water and dawn dishwashing liquid, grabbed a clean rag and started cleaning all that residue off the teeth on my rear aluminum pulley and the belt. Squeeking belts (fan, drive, etc.) are usually caused from dirt or contamination, as well as being too tight or too loose. Guess what? The squeeking on decel was gone! Kept it good and clean from then on and had very few issues. I also found on long trips that if a squeek developed I could use a chunk of parafin wax and rub it on the belt-presto!-no more noise.
Hope this helps, Kelly