Perhaps the other ideas, like EPDM or fiberglassing the plywood..
....I have a large roll of EPDM in my garage (1/8 inch thick). I bought it for emergency repairs on our TPO roof (yep, you read right: EPDM material to patch TPO, using special sealant). I've already applied 2 patches over the years (from a 10-LB tree trimming blade head that dropped over 20 feet onto the roof of our camper, creating a pin-hole in the TPO). The 2 inch by 1 inch and 1 by 1 inch patches are holding well after 5 years.
Anyhow....the reason I mention my 150-foot roll of EPDM: I've been experimenting with this rubber material for a few years, and it just seems like it would be an excellent cushion/barrier under either a metal roof, or under the TPO itself (!)
Wood-shedding this idea further: it may be (it MAY: but do your research 1st with the EPDM manufacturer) an excellent intermediary between say a marine plywood roof wrap, and an aluminum skinned roof. Both as a sound insolator/isolator, and a plywood decking protective barrier and with a slight thermal break property.
I'm not partial to the cheap c%*p RV makers use as aluminum corrugate (it is so thin, it is almost like a pie crust plate). Why don't you source a roll of real aluminum like Airstream uses ???? Far more robust.
S-
