Ian, caravans are not subjected to wind tunnel testing, after all, they are about as aerodynamic as a brick.
If you have the corner steadies down firmly, in theory it shouldnt rock too much, unless your on soft ground, in which case the force of the wind will push one side into the ground somewhat, and allow the van to rock in ever larger amounts.
When it comes to what wind speeds, a caravan is the same as a high sided vehicle, if its too strong for them to venture out safely, its too strong for a caravan, so your looking at winds of about Force 7 and upwards, depending whether your protected by something like high hedges, or not, and wheteher your travelling through a place that funnels the wind, like through a valley or similar, or going through a place affected by cross winds, like costal roads and such like.