I agree, however the sequence of events is important.
An "inventory" of the vehicle is not intended to be a search, and is invalid if the officer did not actually tow your vehicle. Was the vehicle towed?
Searching under panels and upholstery would be beyond the scope of the inventory, however if the officer first found suspected methamphetamine it would enable him to look deeper, essentially anywhere methamphetamine might reasonably be found.
This is definitely something to discuss with an attorney after reviewing the officer's report.
With the information you gave it is easy to see how the officer likely can justify the search without a warrant, however there are ways the officer could have gone wrong too.
You need someone to review the facts who has access to the details. Lawyer up.