We learned the hard way a few years back not to rely on the x-chocks (instead of ground chocks first) to prevent rolling of the tt when unhooked.
We had just pulled into a cg after a 7 hour drive through summer heat with our new tt. DH put X-chocks on both sides right after parking the tt (instead of using the ground chalks). A few minutes later, we were unhooked and heading in for a cold beverage. All of a sudden, a half hour later we hear a "pop" and the tt lurched forward just a tiny bit (we were parked on a slightly slanted site). We ran outside to see there was pressure pushing on the tongue jack. Took a look at the x-chocks and saw they were loose. DH hadn't considered that the tires were swollen from being on the road. Lesson learned. Chock the tires first with ground chalks and place x-chocks after unhooking, then tighten down x-chocks again after at least half an hour. The only reason we use the x-chalks now are for:
-reducing movement in tt while parked
-as a deterrent to slow someone down who may want to hitch up and steal the tt (we use the little pad lock thingies to lock them).