I may have posted about Monopan before, I've never had the chance to use it myself. However they have built trailers out of it - walls and roof.
Translucent, not transparent.
MonoPan :: About MonoPanŽ
Bodies / Chassis - Neptune Auto
I've not operated a big rig before, but in my younger days I emptied out a few of these trucks before. A little natural lighting makes the job easier in my opinion.
Oh, and this stuff is supposed to be light weight, meaning more payload or fuel savings.