[quote="GPW"]Wouldn’t thicker AL sheeting do the job as well , without the potential for Rot ... Sorry to be a skeptic , but we’ve had some bad experiences with unsealed plywood on recreational vehicles, even inside ...
Seems like the Best job would be to spray the metal roof (and walls ) with a good 2 part foam that would not only strengthen and insulate , but also not rot ... A process well proven in our old Kustom van conversion days...

Granted it is more expensive than a thin sheet of cheap plywood ... but if the manufacturer skimps on the metal thickness of the roof, a little plywood isn’t going to do all THAT much ...
We’ve seem many many CTs around here and some , the better constructed ones, do just fine ... the poorly made units becoming “shabbyâ€