In my recent US travels, shuttles have largely been accessible, especially when all rental companies are offsite and a central contractor provides the shuttle. Like DeafFlyer, I've asked rental companies to bring the car to me at the terminal several times with no problems.

The real problems are getting a car or van that can accommodate the scooter and loading/unloading the scooter. I can load my own wheelchair into a sedan, but have occasionally wanted a larger vehicle and was unable to rent one with hand controls.

I'd stick with the bigger companies (Hertz, Avis). I've find the budget companies not nearly as accommodating or even familiar with disability needs.