Here's my story - bought a new 1992 Dodge full size conversion van and part of the Mark III package were aluminum wheels. After a few years I had the mystery leaks like you guys. Fought with it for a year or two. I have a tire changer, so dismounted one and saw that where the bead of the tire touches the rim was very corroded, even though the part of the wheel that was visible was good. The corrosion produced a white powder as often seen on aluminum. I cleaned the wheel, cleaned the tire and thought I was good. Eventually it came back, but at least I knew what it was. Cleaned the wheel again, clean the tire again, this time used the tire slime-goo-sealer on the bead. It's a relatively fast drying liquid that helps the tire slide onto the rim and helps the initial air sealing to pop the bead on, but also provides an airtight seal over time. Never had a problem with those wheels again, and eventually the outside looked pretty bad, too. So if you are remounting or getting new tires, ask for the tire slime-goo to be used. It doesn't hurt anything, can only help, and maybe they use it anyway.
Edit: This is "Tire Bead Sealer" made by Camel as used in tire shops. It is black, dries to very thin rubbery coating. It is not the green "tire slime" that is used to seal punctures in bicycle tires.