I always use standard wheel chocks x4 with my X-chocks. Your wheel stop instructions even tell you to do so. If you didn't, you didn't follow the manufacturer's use instructions. Thankfully, no one got hurt and everyone can now laugh about what happened.
http://ecx.images-amazon.com/images/I/61YOmdKFaqS.pdfCamco Wheel Stop Instructions wrote:
CAUTION: This product is intended to help stabilize a recreational vehicle and is not intended for use on heavy
commercial type trucks or trailers. Always use brake and wheel chocks to safely secure vehicle. Never place
yourself under or behind an unstable vehicle.