You can rent at the parks, but you can't reserve them in advance - and they do run out as the day goes on - although on two different days in November 2005, I was able to get one mid-afternoon. Why did I need to do that? Because the off-site rental I got lost power, despite being fully charged each time. The second time, I ended up "paddling" through DCA using a crutch, to get to wheelchair rental

.
At any rate, you can rent from off-site companies. There's another recent thread in this forum that recommends a company that starts with a "D". I'll see if I can find it. The advantages are, she would have the
ECV 24 hours a day; and it's a long walk from parking or bus drop-off or the main road, or even two of the three on-site hotels, to the parks.
Okay, here's
http://www.disboards.com/showthread.php?t=1520263 the thread - and the name of the company is Deckard. Good luck!