Karen,
Yes, we do currently have three wheelchairs and stay at FW for three weeks every year around Christmas/New Years. Lately we've also made a shorter summer trip since we have AP's and we might as well use them.
We have had as many as four wheelchairs and a stroller. That many does cause some problems for Disney transportation (not to mention pushing that many), but they still have one bus capable of up to five chairs that can be called on special order. You have gotten some very good info here already so I won't repeat everything that's already been said.
We never drive to any of the parks, but make an occasional trip "outside" for groceries, medicines, or a very occasional hospital visit. Arnold Palmer Childrens' Hospital is in Orlando for anything major and there are care centers and first aid stations around for lesser problems.
Because of caring for the kids, most of our transportation to and from parks is not during the heavy travel periods, so crowding on busses is not usually a problem. You will need to take one bus to the Outpost and transfer to a park bus, especially for AK, DHS, DTD, and water parks. We have occasionally had to take two busses to the AK and DHS parks, but the newer 3-chair busses make this less likely in the future. We use the boat to MK and if we're going to EPCOT, we use the boat to MK and the monorail to EPCOT. Coming back from EPCOT we usually take the monorail to the TTC then take the ferryboat to MK and either go into MK or take the boat back to FW. This gives the kids more variety and our oldest loves the ferryboat.
When we ride the busses back from parks we go to the appropriate bus stop and line up street-side of the normal queue where the driver can see us as the bus approaches. When I catch the driver's eye, I hold up fingers for the number of chairs in our party. On VERY rare occasions we have had to split our party or wait on the next bus. We have on a couple of occasions gotten the snide remarks from somewhere in the crowd about preferential treatment, but we ignore the ignorance of the offender. If they had to live a day in the conditions of our kids I'm SURE they would never repeat those remarks. MOST of the time we get great support from the CM's and the other visitors.
We have children on feeding pumps, oxygen, pulse-ox monitors, suction equipment, nebulizer breathing treatments, one trached, etc. We have been to FW for about 12 years and have never experienced a power outage there. If we did, we would just fire up the genny for what would most likely be a short term use. And as someone said, at least some of the equipment had battery backup. We have never used a ventilator, so I can't speak for that, but we have seen a few portable units on wheelchairs in the parks.
We tow a small cargo trailer for the wheelchairs, extra oxygen tanks, formula, golf cart, etc. While at the fort, we keep the wheelchairs outside under the awning, and sometimes under a tarp if rainy weather threatens.
For the size of our children we have installed a chair lift on the side of our MH, but you should have little trouble moving yours as needed. FW has great sidewalks and streets, both around the campground and in the loops and campsites. There is a lot to see and do there even for the special needs children to enjoy at whatever level they are capable. And in case you need a pet loop, contrary to a previous posting, the 300 loop (preferred - pet loop) does have a bus stop that is shared with the 500 loop (premium-also pet loop). As posted the 100-200 loops do not have a dedicated bus stop, but use the settlement stop. Other bus stops are shared among a couple of loops which are right across the street from each other.
OK, I have rambled enough, but want to add that we could not travel with our children without our motorhome. And we would never consider staying anywhere at WDW except FW for that reason. To move the necessary equipment in and out of other hotel type resorts (we've had to do this at a couple of motels in the past) would make the trip a whole lot less enjoyable and we would not have the outdoor space, facilities, and amenities to enjoy like we do at the Fort. If you have access to a motorhome, I'm sure your prayers are already being answered. Have a great trip and keep posting here about your preparations and travel!
