Also, you could try for a van with 2 captains seats in the middle instead of the bench. I would think a wheelchair could fit there.
It looks like a lot of room, but it's not wide enough to fit a wheelchair in between the seats (unless it would fit when folded)
We have never traveled with the exact "configuration" of people that you have, but we have traveled with 4 adults, one older child and one child in a wheelchair (when she was little, she also had a car seat). On our last trip, my mom and niece came later in our trip and rode back to the airport with us, so on our return trip to the airport, we had 4 adults (one was my oldest DD), a 10 year old and an older child with a small adult size wheelchair. She also had a large rolling carry on bag for her communication computer.
As others mentioned, the only time you will have to deal with all your stuff is when you are coming from and going to the airport. Since you have 4 adults, your most comfortable option may be to split up like others mentiones; either rent a small car for the days you are coming and going or use one of the alternate transportation options like a towncar or Mears bus.
But, we were able to fit all of us in the van (not with lots of room to spare, but we did fit).
If the wheelchair folds at all, you can make it more compact and it should fit next to the middle row of seats. We sometimes take the wheelchair apart as far as it goes (pop off the wheels, remove the seat and back) and then stow the pieces wherever they fit, under seats,etc. If it doesn't fit there, you can place a suitcase standing up in that space and/or the stroller (depending on how small it folds up). If the wheelchair doesn't fold, then you need an extended van (like Grand Voyager) to have a wide enough space behind the back seat. That affects your ability to carry your other luggage. The one thing we have done in all our years of going to WDW is editing down what we take. The first years we went, we took much more stuff than we needed. So, take a look at what you are taking and you may find you don't need so much luggage. Then look at the luggage itself. It may be easier to carry 2 big bags, but it will be harder to stow them in the van.
