This won't be an easy job - I've done aluminum sections on aircraft before, but never anything that size. Ask the customer how good of a finish they are willing to pay for, as you can turn that trailer into a perfect mirror but it will cost well up in the hundreds if not into the thousands.
Your first step will be to wash the trailer. I like to use a pressure washer to blast everything off the surface. If you are willing to protect yourself and take the risks, a hydrofluoric acid Aluminum Brightener will burn off any oxidation and leave a nice surface to work with - but if you get *any* of it on you you will need a hospital visit at the least. You'll want a mask, gloves, good boots, and a one-piece full body suit to protect your skin.
To polish the trailer you will probably want to buy this kit:
Rolite Polishing Kit .
The first step will be done via rotary and wool pad. Don't be afraid to cruise at 2000+ RPM to remove defects and oxidation from the surface. This is a very messy step - there will be polish and wool fuzz everywhere. Use a pressure washer to clean it off and get into the rivets, etc.
TOL's website says to apply the second step via terry bonnet - I like a light wool pad instead. Use the rotary - drills are useless for something the size of a horse trailer.
The third step is final polishing. You'll need this for a show-quality shine, but you might not want to do all 3 steps unless the owner is willing to pay for it. Do it with a Cyclo or rotary and foam pads.
I would charge 750.00 per step on a trailer that size. I would plan for a full day per step or more. If you are going to go all the way, plan on 3 days plus to knock it out. I'd probably wax it with Collinite.
If I were going to do it myself, I would just try to sell them a one-step cleaner wax via rotary for 450.00 or so. Collinite makes a metal wax you could apply with a light cut wool pad, and it would give a fairly good finish at a fraction of the cost and frustration of doing it right. You could do the cleaner wax in a full day if you work fast.