Mold On Paint

On my 75 F250......first, let me describe the paint. It's original. It's that old "Wimbledon" white. Kinda a eggshell white. The vast majority of the paint is actually still good enough to shine up pretty decent. My problem is black mold. I can wash the truck GOOD and several weeks later, regardless of what I try, it comes back. I don't park directly under trees, but there are pine trees around where I park. Kitty's car doesn't get the stuff on it. It's just dirty. lol It's also red, if that makes a difference. I've tried it all. Good name brand car wash, STRAIGHT Purple Power, 50/50 water and bleach, Simple Green. The paint is oxidized, because when I do wash it, the wash bucket turns white, but dammit, something has gotta get this crap off and keep it off.