Hi and welcome.
Some people have had luck with the park holding the
ecv for them, but I don't think you can count on it, since other people have posted they were told it would not be held. We have not rented an ecv, since DD has her own wheelchair. Hopefully, someone can give you a better answer than I can.
The only way you can make sure you have an ecv when you want one is to rent from one of the off-site places. Here are the ones that DIS posters have reported renting from:
Care Medical:
http://www.caremedicalequipment.com/
Walker Mobility:
1-888-726-6837
www.walkermobility.com
RANDY'S Mobility is in Kissimmee 407-892-4777
http://randysmobility.com/
Colonial Medical
http://www.colonialmed.com/about_cms.html