Tell the truth . . . you have a team of trivia geeks working with you. 8)
The National Park was more involved, and I'm still not happy with the documentation. I wasted a lot of time on the National Park Service site. (A lot of PDF junk on there, that I didn't bother to open.) Along the way, I found the first National Seashore (let's wait and see if that turns up on the appropriate state list). My guess is that the actual winner will turn out to be a War Department purchase of a battlefield site before the NPS took those over. For example, Kennesaw Mountain started with the purchase of a small tract of land for a memorial erected by the State of Illinois (1899) was transferred to the War Department in 1917, and to the National Park Service in 1933. Most of its growth occured while under the War Department. http://ngeorgia.com/travel/kennesawmtn.html
But I'm missing the link to show who paid for the expansion.