ABH19,
The stiffening added with interior paneling is minimal. I've seen Featherlite car haulers that had none, just bare metal showing.
I took it out because I wanted thin Birch plywood on the interior. Featherlite has that "Wonderwood" OSB made with extra resin/glue and compressed (like MDO in plywood). You could hit it with a hatchet and it would not penetrate. Good stuff for a contractor's cargo trailer. Very durable. Not what I needed.
My trailer is stiffer now than from the factory because I panelled everything including the ceiling, put rigid foam tightly in every hollow and used hardwood crown molding.
For Featherlite trailers, YES on the wall posts being connected to the roof structure. It's all aluminum welded. (And all the steel trailers I've seen are welded there also.)
Hope that answers your question.
Prem

My goal...
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...is to live in a trailer.