There is a big difference between Europe and North America in the weight and balance of towable RVs, and the way in which they are towed. The tendency over there is to balance for a light tongue weight, and to tow slowly enough that a trailer so balanced will not reach the speeds at which the combination becomes inherently unstable.
A vehicle manufacturer here, in developing safe tow ratings, has to assume that someone towing a trailer will try to tow at at or above the general speed limit, no matter how high that limit is.
I see that Melissa linked to the relevant article.
