I like the idea IceeofMN had regarding bowing the Luan top just a little. I bet makeing the center of the cross members 1/2" higher in the middle would make a big difference and I think that you could get by with that using Luan, especially over a 5 1/2' radius.
Of course that brings up other issues like filling the void with insulation (Great Stuff?) and the angle at which the wall meets the side.
I decided that a 1/4" plywood - 3/4" pink foam - paneling bonded sandwhich with 1 X 4 vertical stringers would make a strong, lighweight wall. I will bond and seal the roof seams with epoxy and fiberglass tape. 1/2" ply on a 2 X 3 framefor the floor and the sides will attached to 2 X 2s run the length of the floor, offset inwards so the wall sits on the trailer.
I have also built with Luan in the past (sailboat) and the combination of stressed or sandwhiched Luan and epoxy/fiberglass is strong. Even thought about using 1/4" (6mm) Luan for the sides.
EZ