From your description it really sounds like off gassing. Mine was built in the days of Heavy formaldehyde smell from all the glues. For years when going in after a sitting in storage I would have to hold my breath and go open a bunch of windows and let it sit overnight. It was so bad I would get a sore throat from smelling it if I didn't air it out. Once aired out the small was there but very faint.
On yours I would put covers on all roof vents so they can stay open 24/7 That should help air it out and reduce the smell while in storage.