Do you folks have problems with acid rain up there ?
Are there any power plants, etc close by that could be putting something into the air ?
Do you live near salt water ? Could the wind be blowing some kind of constant salt mist onto your truck ? (maybe somehow reacting with whatever wax or chemicals you use)
What about fertilizers ? Any possibility that any could land on your truck ?
Are there planes applying any kind of fertilizers or pesticides ?
Do your neighbors have the same problems ?
I still think this is chemical related.
Tim.