Your assumption above is incorrect. To understand one needs to read the tax law and the IRS tax case. The income OPA receives from Seacrets lease is non-taxable, but not because it is "open to public." The Seacrets income is non-taxable because the tax code makes rental income to a non-profit like OPA non-taxable. Also, the money for OPA income for Beach Club parking and what Seacrets pays on their lease are already separated.

OP is going to have to dig out all paperwork for all expenses incurred on the parking lots including attendent salary since 2005.

OPA already keeps track of expenses related to the parking, so I don't believe any special or substantive effort is required.