My understanding is that the resort wheelchairs are not meant to be taken from the resorts, though I know many people have reported taking them to the parks.
The ones at the resorts tend to be very old and in poor shape. They are not adjustable and many people have reported that they are very hard to drive (bent wheels, broken footrests, etc)
You cannot reserve them ahead of time, and there is no guarantee that you can get one.
If you require a wheelchair, then you should either plan to rent one in the parks, or rent one from off-site. That way should you have a problem, then you can get another one.
there are phone numbers to rent off-site in the FAQs