National has a shuttle service available onsite at WDW. I believe you have to call for pick-up. If I understand correctly, you are looking to drive a minivan from the airport then drop it off at the National office on WDW property?
No regular Disney buses go to the National Car Rental office on sige, which is just beyond the MK parking lot exit (you are heading south) and before the lanes re-juxtapose themselves with the northbound lanes of World Drive (main drag from 192 to MK).
If worse comes to worst, one of the authorized drivers could drop off the car, then walk to the MK lot and take a tram to TTC and then get back into the Disney transportation system.
The transportation that is provided and that you telephone for has quite limited hours.
Dreams Unlimited Travel is committed to providing you with the very best vacation planning experience possible. Our Vacation Planners are experts and will share their honest advice to help you have a magical vacation.