It is an OEM (Ford) aux input for factory radio. It isn't used by any RV upfitters so they pop a cap plug in it.
At one time you could "port" an iPhone to the aux but iPhone changed the interface of their phones too many times over a few years therefore aftermarket gave up on supporting the Apple.
Ah, shootskis. I forgot all about that. A year ago we rented a 30' CruiseAmerica Class C that had one of those. It worked. It was a stereo headphone jack style connection. We took along an eighth inch patch cable and successfully streamed audio from my phone through it.