Hello, Kathy,
If I were in your shoes, I would rely on
Disney transportation. In fact, we have never rented a car at Walt Disney World as we like the simplicity of using Disney transportation and find it very easy to use. We have also traveled with my mother, who is in a wheelchair and cannot transfer out of it. Disney drivers were always the best when helping my mom, and I know they do the same for other guests who use ECVs and wheelchairs. Prior to your trip, you may want to look over information Walt Disney World has shared for
Guest with Mobility Disabilities, which includes information on Disney transportation.
Since you are staying at the Campsites at Disney's Fort Wilderness Resort, you will be able to take a boat to the Magic Kingdom (unless water levels drops making it difficult to wheel the ECV onto the boat; buses will be available if this happens) and a bus to other theme parks. Since it does take additional time to load an ECV and safely secure it on a bus, you may want to always allow some extra time in your plans.
Wishing you a great trip; please let us know if we can help again.
-Kara