Oh yeah, insulation is absolutely needed. I have 2" insulation in my ceiling. With a 13500 BTU roof top unit. With the walls still bare metal, awaiting insulation, the interior was OK. About 78 in there in mid day in August when it 98 outside. Nice and cool at night though. I'm working on doing my walls this weekend. I picked up enough 1" and 2" foil-backed white styro insulation to do all the walls for 3 cases of Budweiser. Heck of a deal. I still need to paint my roof white though...
If you don't get your trailer pre-insulated, you might want to build your structure out to 2" and use the thick stuff. I went to a screen-porch place and they sold me a bunch of 1" and 2" aluminum to screw in as standoffs. Worked great!