Some trailers do have coating on the outside and the acid will strip it off, best to check the manufacturer. Our trailer has a big sticker on it "NO ACID WASH". If you do an acid wash at the truck stop it opens the pores of the aluminum and "peels" a layer off. Think of an exfoliation on your skin.
If not kept up the acid wash will oxidize the trailer horribly and turn it yellow. Usually after an acid wash it is buffed and a mirror shine is put on it. Absolutely beautiful but it requires a HUGE a mount of maintenance and money.
They do make pH balanced trailer washes you can buy and use at home for aluminum trailers.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.