We've stayed in van parks with our soft floor camper, and only once had a situation that was just too hard. Even though you've got that booking, ask again when you turn up of there is anything else available, as sometimes they keep a bit of grass for last minute arrivals.
There are two things to consider if you need to setup over the slab; how you'll peg it down and damage to the floor. I have a used a Jerry can full of water in the outer two corners on occasion when we couldn't get a peg in. I have a large ground sheet normally used in the annex which I have laid under the tent to protect the floor from the concrete (or even worse is those small pebbles NPWS use).