Pressure washer and you'll want an aluminum brightener. If you have a truck stop, truck parts/dealer or a pressure washing supply shop near by they should have some soaps and cleaning chemicals.
On the ingredients for the aluminum brighteners, look for Hydrochloric Acid. Try to stay away from Hydroflouric (not as safe to work with). Wear gloves, glasses and try not breathe it. Follow instructions for dilution rates. Buy a cheap Home Depot 1 gallon pump up sprayer..apply, give it some dwell time and rinse off.
*Note..some brighteners will "brighten" and some will "whiten"...also keep any brighteners from polished aluminum.