How cool, we've talked about a trailer model for the local farmers markets, pizza and bread.
Laminating would probably be a better way to go. Den-Den's suggestion made my mind wander to doing it like a West System hull. Laminated veneers to form entire relatively lightweight sections rather than discrete pinned frame members covered by a panel would give you better strength to weight and I think better durability.
For the ribs that form the roof ash was the old choice, its also the frame in canoes, baseball bats, hockey sticks, etc. It takes impact and bending loads well. It can be sized to a better strength to size/weight member than something brittle but lighter like cedar. What I'm proposing is those veneer panel stiffeners are glassed in while making the panels.
I read an old Forest Products Labs paper from the '20's where they were concerned that the automobile industry was going to wipe out the ash trees for building car frames. They were testing alternatives and had decided red maple would be the next best choice to move to. We soon went to steel car frames. My 35 dodge was the first year of an all steel truck. During our lifetime we've seen maple baseball bats come into play.