I have a 2014 Toyota Sienna. I have a X19H Hybrid that has a dry weight of #3600. At first glance, many will say that I am overloaded because the stated capacity is #3500. I have done a lot of research on this vehicle and its towing capacity. I have reached out to a few custom towing experts and read numerous peoples' account on towing with this vehicle. I encourage you to do the homework on this and come to your own conclusions. Do not read this post as an endorsement for towing above the 3500# mfr limit. I am simply telling you about my set-up and my experience towing with this vehicle. I spoke to the people at Can-Am RV in Canada (http://www.canamrv.ca/towing/
you MUST watch the Caravan towing a 34' Airstream!) as they do many custom towing set-ups. As stated by them, they have installed the following on many Siennas: Class III Receiver with a Weight Distribution Hitch, P3 Brake Controller and an AirLift 1000 air suspension. The thing you do need to watch is the tongue weight. The X19H is right at the limit, but the WDH helps keep everything level.
A couple quick points that I found on the Sienna. Toyota (and all mini-van manufacturers) state the 3500# limit for a number of reasons, none of which have to do with its capabilities (again, some research you'll see why). The Sienna has the same motor and the same transmission as the Toyota Highlander. The Highlander is rated at 5000#. None of which includes the passengers, the max weight in the tow vehicle includes passengers (according to the manual).
Now, I know that some will say that the vehicles are different in structure. They are, but they are both uni-body construction. The Highlander does not has a truck frame. As Can-Am pointed out, the Sienna actually makes a better tow vehicle due to its wider wheelbase and lower center of gravity. The frame can handle it.
I have been towing our camper all summer on many kinds of roads. It tows very well. On the highway (flat) at 65 mph the rpm is around 2700. I have yet to see the temperature go over the half-way mark. All 2014 Sienna's come with the towing package from Toyota. So it has the transmission and oil cooler right from the factory. But, the brake controller will have to be wired by a professional. The Class III receiver is a bolt on.
I have attached the pictures of the AirLift 1000 Air suspension. This helps keep everything level when loaded up.
Again, understand that you have to make the decision yourself. I am just telling you about my experience with the Sienna. I feel very comfortable towing this camper. I have attached a couple of pictures. One of the AirLift and one of the camper hooked up. Note: This picture was prior to adjustments to the WDH and supension. Once adjusted, I only lost about 1" of height in the rear.