To wash it, just use clear water, Walbernize products or a mild soap. This will get the dirt & grime off, but if you what to get rid of the oxidation which makes it "dull" you will have to polish it which is a whole 'nother ball-o-wax. There are many different opinions on how to polish an Airstream, you can peruse the exisitng threads here to get an idea of the method you would want to take on then ask for help from those who have gone there before you ~
Good luck in whatever you decide to do ~ most of all, have fun on your first trip!
Shari