1. How far in advance can you book a cruise?
You can book now for June 2017 sailings but the dates and itineraries have not been released yet for 2018.
. What state room fits a family of 6? (2 adults, 2 teenagers, 2 children)
I would go for a verandah stateroom and then put the teens across the hall in an inside cabin.
3. If I did 3 days at Fort Wilderness, then the cruise, can I leave my camper in Fort Wilderness parking lot?
I would imagine that you could park the camper in the huge parking lot near the horses and where the busses come in while you are gone. People take Disney transportation to and from the port all the time so it would make sense that guests who have stayed at the resort and took Disney transport to the port would be able to leave campers as well. But of course, I would call and ask the resort directly.
However, you could always take the camper out to Port Canaveral and park at the terminal too.
Here is the website for pricing ideas to park at PC :
http://www.portcanaveral.com/Cruise/Directions-Parking
4. Where can I get average prices at? For early June?
You could go and look at the prices now for June 2017 to get a clear idea of how much it will cost based on cruise length of your choice. For example, 3 and 4 Night cruises are on the Dream out of PC and 7 night cruises out of PC are on the Fantasy. If I recall correctly, there is a 10 night and an 11 night Southern Carribbean in June that is on the Fantasy as well.
5. How many days is the cruise?