We are members of DVC at OKW and have a DD who uses a wheelchair. We rent a van and sometimes drive, so we have experience with a wheelchair on the bus and also with handicapped parking areas at the parks.
The bus drops you off right at the gate at the MK, so that is a better option than driving. If you drive to MK, you will need to take a boat or the monorail to get to the same place that the bus lets you off.
At Epcot, the handicapped parking is closer to the entrance than where the bus leaves you off, but we have had the experience of the handicapped parking being full. AK's buses leave you off a bit closer than the handicapped parking (plus, we have found it hard to get a handicapped parking spot there too). At the Studio, the buses board just in front of the handicapped parking area, so it is a bit closer to ride the bus.
At any of the parks, you can request a place at the end of a row, near where the tram picks up. That is actually the best way to avoid walking (except at MK) since you have a short walk to the tram and then are dropped off right by the gate.
