I don't think South Alabama went through the full range of classifications, playing a hodgepodge of teams at the outset. GA Southern was 1-AA from its re-start. James Madison skipped DII.
Jax State was NAIA before the heyday of Alabama DII teams in the 70's and 80's (Jax St, North AL, Livingston, Troy). I don't think they were ever DIII.
You have to go back a ways to find programs starting as DIII's, and DII's still being a viable stepping stone if you truly want to get to DI.
I'm going to go with Central Florida.