I have the exact same TV (2006 Sienna) and a 2720SL.
You can do this, the Sienna tows very nicely...but I'm not tackling mountains, I'm in the flat, flat midwest.
You do NOT need to worry about "tow packages" with this van--the engine already has all the parts you need. Hitch, brake controller, wired bargeman are all that need to be added to the TV. WDH is required, sway is optional, but probably unnecessary.
If you find the sag too much, air lift bags apparently work wonders. I have not needed such, for whatever reason, I've not noticed an issue with sag.
Feel free to ask questions, I'll be happy to share my experiences
