I treat my open trailer pretty much like a nice car, keeping it clean - pretty easy for me, as it mostly lives indoors except Summer, when it's covered outside when not in use.
There's generally not a lot of paint on the enclose trailer panels, so a detailer shouldn't attack the painted areas like he would an old car.
What it needs cosmetically depends on existing condition: paint smooth or not, aluminum cloudy or not...
I'd start with a good look underneath to make sure it's decent, not rusty, and undamaged. Check welds & panel integrity, too.
Wheel bearings, tires, electrical, power tongue jacks & such deserve an inspection and whatever servicing/attention they may need.
Polishing the paint & aluminum, and having a good coat of wax on everything is a good idea, IMO.