The TSP that I used was sold in powder form; mixed 1/2 cup : 2 gallons water, then applied just like any other car wash soap with a brush and/or sponge and rinsed well. The Bar Keeper's Friend is, as said above, a mild scouring powder used with a white 3M scrubbie pad to remove all the chalky oxidized surface.
Went ahead and added the 4th coat yesterday. Don't think I really needed it, but I figured that it couldn't hurt and would give an extra layer of protection. It only took about 30-45 minutes to apply. I still have more than 1/4 of the gallon left; plenty for 1 or 2 touch-up coats next year!
Yes, I also applied it to the window frames; mine are brown. They were faded & oxidized as well; they now look new. My decals weren't peeling, but they are cracked & checked in some areas. I left them as is; they are still cracked & checked, but they are now a deeper color and shiny, so they still look 10X better than they did.
Again, I can't get over the difference. I've just increased resale value of my MH by at least a few thousand dollars (not that I'm planning on selling, but...). I keep walking outside to look at it just because...:)