If you want to renew the finish on your painted aluminum trailer, the best thing you can do is remove the oxidation. The easiest way I've found to do that is with either a phosphoric acid based product or using TSP. Lately, I use Jasco Prepaint Cleaner and Conditioner. I spray it onto painted surfaces with either a tank sprayer or a bottle sprayer.
The drill is to clean first, apply the Jasco and let it sit about 10 mins.. I then rinse it off using a scrub brush, lightly. It will remove almost all oxidation and the surface is then ready to paint or wax.