OK, before I get into this, here is the "Legal Disclamer" form the Toyota web-site
Before towing, confirm your vehicle and trailer are compatible, hooked up properly and you have all necessary additional equipment. Do not exceed any Weight Ratings and review all instructions and cautions from your trailer hitch manufacturer and Toyota vehicle Owner's Manual. Note that the maximum amount you can tow depends on the total weight of any cargo, occupants and available equipment in the vehicle.
Additionally, the Toyota web-site does not list the Gross Vehicle Weight Rating (GVWR) or the Gross Combined Weight Rating (GCWR) for your vehicle. It states:
If you would like further towing and vehicle capacity information, please reference Trailer Towing in your vehicles Owners Manual index for the appropriate page numbers. You may also contact your local Toyota dealership for weight ratings and additional required equipment.
.
Now, as I don't have exact numbers I'll make some up to use as an example.
Lets say your vehicle has a GCWR (remember, this is the maximum of both the tow vehicle AND trailer) of 8,000 lbs. You subtract from that the weight of the Sienna (4,165 empty), passengers (500 lbs) and luggage (400 lbs). You are now down to 2,935 pounds that your trailer can weigh.
Now, with all that said remember you are talking about maximum numbers. You really don't want to push the limit.
I'm sorry, but my gut feeling is that you would be pushing the limits of the Sienna.
Another thing to keep in mind is that your vehicle looses power at altitude. Unless the engine is turbocharged, it will loose power with altitude. The typical power loss is 2-4% per 1,000 feet above sea level. So you won't notice in the first few thousand feet, but if you reach 8,000 feet, you cound be missing 20-30% of your horsepower at the very time you need it most.
Here are some pages/sites you may be intersted in:
Towing and Hitch Information (from my web page)
NCCNEPHCOFF (Nick-Neff-Cough) Northern Californai Pop Up Club
Southern California Popup Campers (SCCampers)
Pop UP Times Magazine (the only magazine for Pop Up Campers)
Check you owners manual in the towing section and let us know what the exact numbers are for the Sienna.