How Often Do you Get Skunked on FCFS Camp Spots?

I start making reservations in November just so we don't get caught. With a little bit larger trailer, would hate to drive around looking for a spot. We did make a reservation last year then I tried to move to one of the FCFS spots that was not occupied, but the camp host said I would have to pay for both spots. I have not gone to an area just because I am not sure if the campsite would be full and didn't want to chance it.