Seems there are a lot of misconceptions here. I also own and tow with a 2005 Toyota Sienna, and happen to also live in Houston. Here are the facts.
According to CAT scales my Sienna weighs 4300 lbs. It is fully loaded. Most weigh around 4100 lbs
According to Toyota:
GVWR= 5700 lbs
GCWR= 8800 lbs
Tow rating= 3500 lbs
The true payload is anywhere from 1400-1600 lbs (more than most 1/2 ton crew cab trucks) In most Sienna's you can pull the full 3500 lbs and still add 1200lbs of stuff (people, cargo, tongue weight) to the van.
The wheelbase is 120 inches, which is longer than a Ford Excursion
Our Sienna has over 108,000 miles on it now and has towed the entire time with zero problems. I tow a boat with it that weighs around 2000 lbs, and also a full size 27ft camper that weighs 4000lbs fully loaded. We do not use weight distribution with the boat- no need.
We do use weight distribution with the camper, as it has a 500lb tongue weight. The hitch is NOT bolted to sheetmetal...it is bolted to what can best be described as looking no different than a frame. (I know it technicallly is not, but the OP can look under his van and will see the same).
We have had no problems towing either the trailer nor the boat, and have NEVER had any problems getting our boat out of the water. If anything it is the people in rwd trucks that have the most problems at the ramps.
Here is a pic of our camper at Lake Somerville. Sorry, no pic of the boat