It's not necessary, but it really helps. Especially if you are going to be at the Fort for 3 months. However, as others have said, I would really look into renting one outside of Disney and bringing it in, it would be cheaper, and there really is no reason to rent from Disney for that amount of time (I've never even heard of anyone renting for over a month...lol).
You can get anywhere in the Fort by walking, however, if you're going to be there for 3 months, you might get sick of walking after a while (depending on where your campsite is). If you're going to be up near the front of the Fort, it definitely will be a little bit of a walk to get up to the docks/trading posts/etc...
You can get anywhere in the Fort by walking, however, if you're going to be there for 3 months, you might get sick of walking after a while (depending on where your campsite is). If you're going to be up near the front of the Fort, it definitely will be a little bit of a walk to get up to the docks/trading posts/etc...