A Hidden Hitch receiver, strengthened in some mysterious manner. An Eaze-lift Elite hitch, a pair of Eaze Lift torsion bars for weight distribution and a pair of friction anti-sway bars.
The Sienna has had larger oil coolers fitted by Can-Am RV and there is a Prodigy P2 Brake Controller installed as well. The deal came with a pair of McKesh extension mirrors, too.
If you do the math with the bald figures supplied by Toyota then this rig shouldn't work. However, with the modifications made and a careful disregard for Toyota's tow rating then we really do have a good towing vehicle for this trailer.
I don't have the figures to quote here but Can-Am RV has hundreds of happy customers towing Airstreams with all manner of cars, SUVs and Minivans (and trucks, of course). Indeed, Can-Am's business IS towing and they really know what they are doing; I trust that they do their math before they let me loose, then I and other customers put their good work into practice.