US residents can book a maximum of 14 consecutive days on a single reservation. Longer stays require multiple consecutive reservations; if staying at the same resort, talk to the resort to see if you can keep the same room/lot (you should be able to, as assignments are typically made in the last few days before check-in).
I was just talking to a Disney agent about this. I was telling her if we won the big lottery I promised my kids a year at WDW. She said there are people with an RV that will stay for a couple months at Fort Wilderness,so apparently it can be done. I would call direct though.