There is a parking lot at the entrance to FW, including an overflow lot. Just park somewhere in that area. Watch the signs though as some spots might be specific for check in/out, but I cannot remember for sure.
As the PP said there is a parking lot at the front of FW. You could also park at the MK if that's easier for you as IIRC you still have free parking un until midnight of check-out day. That way it's not car to the internal bus to the boat to MK and the boat back to FW and internal bus to the front to you car, it's only tram to the TTC and monorail or bot to the MK and then monorail or boat to the TTC and tram to your car. Might give you more park time by a few min which is always a good thing.