They do have some "multiple day" discount offers for wheelchairs and strollers. You pre-purchase the number of days you want and can save $2 per day for wheelchairs over the "regular" cost. There is information about it in the disABILITIES FAQs thread.
I don't think they have the same option for
ECVs (at least they didn't when I got the information about wheelchairs). ECVs are in more limited supply and they might not be able to guarantee one would be available that you had pre-paid for. I do know (thanks to Cheshire Figment) that the "length of stay" option was not in effect over Christmas/New Years when it was very busy.
The buses, monorails and most boats can accomidate wheelchairs and ECVs, so if you rent one from off-site, there are more options than using a car.