I have a buddy who has a swimming pool water delivery business and he uses an product called aluma brite and a hot water pressure washer on his aluminum tanker trailers. The difference is amazing before and after he does this. It is acid based so be carefull. Here is where he gets his stuff.

http://www.sunbritesupplymd.com/WASH..._Maryland.html

I also use the Eagle wheel cleaner with acid with a red scotch brite pad, its tuff in your hands but it works