First come - first served. Arrive by 3pm and you should be OK. You have to pay daily at the machine, credit card only, and display the ticket in your window. There are patrols, and they call the police to any RV without a displayed ticket.
It's first-come, first-serve. It's actually a municipal parking lot with a pay machine -- the only difference from a regular parking lot is that you buy parking time in 24-hour increments (and there are utility posts).