What thickness of Coroplast for enclosing TT underside?

well, the thinner ones is the one most TT mfg use. I'd guess our outback is about 1/8".

I would think 1/2" would be stiffer and less sag, but much harder to work with. the 1/8" stuff can easily be cut with a razor blade and rolled up.

And Idon't think the thick stuff would add much insulation. I'd go for the thinner stuff and foamboard above it.