DO NOT use metal 'Drywall' studs. Anything metal will transfer (conduct) the outside temp right into the trailer.
Do a internet search for 'Thermal Brake' and study up on the subject matter.
As for the insulation a 'Closed Cell' polyurethane insulation would be the best and provide a R-Value of about R-6 per inch of thickness. You can 'spray in' or install sheets of this insulation as I have, here's a link
R-Max and can be found at Home Depot, Lowes, etc.
The nice thing about a sprayed in application is that it also provides an 'Air Barrier' as well, do a search on that term as well.
Trust me, I'm building a 36 foot hauler from the groud up
Boonhauler Project