How would I establish the "original basis" of the property that was inherited 12 years ago (upon death of a parent) and not rented until 2018?

[TomD8]

The cost basis of an inherited property is its fair market value at the time of the decedent's passing.  You might be able to check with the county assessor office to obtain at least a ballpark figure.  Another possibility is to ask a local realty office.  They may have price records on comparable properties that sold 12 years ago.