Pure "sunlight soap", a bucket of water & some 000 or 0000 steel wool (depends on how rough the surface of the aluminium is). Throw the soap into the bucket of water to let it get softer, then rub the steel wool into it. When rubbing it into the surface it will not scratch due the the animal fats in the soap and these fats will leave a non greasy protective film on the surface that will last for a few months. When you wash the trailer after a trip away, the water beads up like a freshly polished painted surface.
I used to polish restored motorcycle alloy and the more you use it the shinier it gets. Do it enough times and it starts looking like chrome. I hope this helped. It must be "pure" Sunlight soap, you can buy it from Coles or Woolies, five cakes in a cardboard box, just like it was sold decades ago.
Gary