There's an easy explanation why you experienced improved stability with a big box on the roof of the car, Steve. The big boxes take the headwind that would otherwise hit the front of the caravan. The reduced wind drag on the van lessens the effect of decreasing the noseweight. As everyone knows, a low noseweight is one of the major contributors to instability, so if the noseweight is not reduced by air drag, the outfit will remain more stable.