Unless it's the fact they don't travel as far in Europe to camp, I don't have any idea why their tow ratings are higher. I will venture a guess on the US 3500lb minvan tow rating however: I have the perception that minivans as a group suffer more transmissin failures than many other types of vehicles even when they are never used for towing.
Someone discribed the problem as a "packaging" issue in that the motor, transmission and other running gear must be small enough to fit between the front wheels and still leave room for the wheels to turn for steering. This neccessitates the use of small, hi-tech transmissions that may not be as durable as those available for rear wheel drive rigs. The fact minivans now weigh about 4500lbs and have ever-increasing HP may add to the existing problem. I'll venture a guess that the transmission in UtahSues HD diesel pickup probably weighs as much as the engine and transmission combined in some minivans.
This is speculation on my part of course but I can offer a couple of facts that support the theory: We kept our Odyssey minivan/2619 combined weight to a few pounds below the Odysseys GCWR and towed for 20,000+ mi
with no problems and our transmission fluid was still pink at 30K. Another Ody owner reported on
http://www.odyclub.com that he was towing a mid-size, slab-side trailer without difficulty except that Honda had replaced his transmission twice (no questions asked) under warranty. Since Ody transmissions run about $5K per copy I assume he traded either his TV or trailer before his warranty expired.