Oh yes they book up.
I was booking IN November for February, and already the basic types of rooms at RPR had booked up entirely. So I could only compare the higher-end rooms to the other resorts (who both had normal rooms available).
I'd book something NOW, while room types and good rates are available. If you want to cancel later, you can. Sure it holds a couple/few hundred dollars (depending on what one average night's rate is during your stay), but if you cancel before 5 days you get it back, and if you go, that's money you don't have to pay again. AND given your time of travel, it's going to get more and more expensive, especially as the basic rooms book up. So it will save you money.
(and if, by chance, you see a LOWER rate after you've booked, you can change!)