Trailers and large commercial vehicles, RV's, etc can be tricky. I've got myself in trouble with stuff like this in the past. They can take forever! 20+ hours!
If the paint is just a little dull, it's probably just slightly oxidized and should correct failry easily. M105 and a wool pad on a rotary will probably make it look nice. You won't get anywhere with a da imo. Try some test spots though, it may take a little less aggressive approach. You could try something like m105 or 205 with an orange ccs pad.
Remember, it's just a trailer and he probably doesn't expect the same type of perfection as a nice car. Nobody is looking for swirls on a trailer.
I would suggest trying to make it look as good as possible in one step and then waxing or sealing...make sure the sealant is compatible with that type of paint.
My most important piece of advice. Charge by the hour on something like this. If you don't do them a lot, there's no way to tell how long it might take.
I would expect that trailer to take a sold 10 to 15 hours...depending on if it has a lot of trim or intricate parts. You probably want to make at least $30 per hour...minimum. You will use a ton of product and probably ruin a couple pads. Just make sure you keep that in mind, and be careful on any graphics or decals! Good luck:xyxthumbs: