Zoltan, I am not sure what the tow limit on the Sienna is, but have you considered the Trail-Mini ???
Bill mentioned a lite weight upright, so how about 1 of these:
http://www.forestriverinc.com/nd/def...hoice=brochure
The heaviest unit is 2500 lbs. dry, and they are less than 7' wide, so that should help with the wind resistance.