According to Hertz's list for Orlando, the Caribe Royale is the closest. It's about 2 miles outside the Hotel Plaza Blvd. gate, if I'm estimating the distance correctly. (I know none of the locations on the list is any closer to WDW, though; I know where all of them are.) Depending on which onsite hotel you need to get to, that could be a nearly 8 mile cab ride back to it.
Can you go with a different company? Nat'l has 2 locations inside the property, and several of the DD hotels have car desks, too (though not Hertz, apparently.)