My understanding that it is first come first serve. However, it's not based on when you make the reservation, it's based on when you actually arrive. Usually a request for a rollaway on a reservation is just that, a request. The hotel can't be certain that you will arrive and if you do that you will still want that rollaway. They make money from the rollaway so it's in their interest to rent them out as quickly as their requested so they make as much money as possible. I would suggest contacting the hotel you choose a few days in advance of your arrival to talk to them about the fact that you want a rollaway. They might be able to put your name on one then.