Citric acid is very popular for doing cattle pots.  

The other good acids are Hydroflouric/Sulfuric mixtures.  

Don't spray it on anything thats polished.

Pressure wash the trailer first. You will get streaking from dirt that is on there.

Making sure to agitate it a bit will help avoid streaking and do not let it dry. Work in small sections.

Dilution for the product I use is up to 1 part acid 100 parts water. I use double mist sprayers for small stuff and a backpack sprayer for the big stuff. It will ruin a lot of sprayers not rated for harsh chemicals though.