The vintage airstream campers came with a highly polished finish then a coat of lacqure was applied to protect the finish
When the lacqure ages and peals off or is removed, the aluminum will corrode and darken. This oxidation forms a coating that prevents futher oxidation or at least slows the process. The problem everyone is having is when the protective coatimg is removed, the aluminum will take a buffing and look good for a while but will began to oxides almost imediately and within a year will show signs of dulling or oxidation.
Once you have removed the protective finish (lacqure in the old days) and polished your unit, a new protective coating must be applied.
Several folks here have recommended some products however I dont have any recommendations.