I flew round trip from my local small airport to Minneapolis (connections in Atlanta) a few weeks ago. I only fly Delta so I can only speak of my experience with them. I booked my tickets online and then called their disability assistance line. I told them that I had mobility issues and that I would need wheelchair service from the time I arrived at the airport until I got to my rental car. I told them I had someone flying with me to help me on the plane and to drive, but that she couldn’t handle my luggage and hers. They said no problem and meant it. There was some one waiting to help me get in the airport, through security, to my plane seat and waiting with a wheelchair for connections. They took care of my carry on and everything. Everyone was super nice and I didn’t have a single problem. I would suggest not waiting until you arrive at the airport to book wheelchair services. It just speeds everything up to have it noted on your booking in advance of your arrival. I also highly recommend signing up for TSA-precheck. It makes the airport experience easier to handle for those with mobility and disabilities (keep shoes on!).