You can request first floor or one of the buildings with an elevator for medical reasons. Since all buildings have first floor rooms, you are almost guaranteed to get one of the first floor rooms. They do fill medical requests before general requests (and most people at OKW who request a floor are requesting a higher floor, not first).
There are not that many fully handicapped accessible rooms at OKW, so requesting one of those gives a lot fewer options. As long as you have a first floor room, you won't have a problem with manouvering the
ECV around the room.
You can store and charge an ECV at Papa's Den in the check in area. There are usually a number of them there. Remove the key if you leave it there and no one can drive it away.