This is what is listed on their site. You can call National's 800# and they can tell you what vehicles are stocked at what airport.
Mini-Van : Pontiac Montana or similar
Vehicle Features list
6-Cylinder Engine
Automatic Transmission
Air Bags
Anti-lock Brakes
Air Conditioning
Power-Assist Steering
Tilt Steering Wheel
AM/FM Stereo
Tinted Windows
Power Windows/Door Locks
Dual Mirrors
Delay Wipers
Just a quick note...... We have rented minivans in the past for their extra room for people and luggage and were dissapointed to find out that they lock the rear seat in the furthest back position and you can't put that much luggage back there at all. We had to upgrade for more $$ to a larger minivan and even then we had to put luggage on the laps.
I have rented a number of minivans from National. They have always had the extended length minivans when I have gone there. The last time we were there, which as last summer, they had 8 passenger Montana's, Ventures, Grand Caravans and Astro Vans.