I'm not familiar with the Toyota Sienna's towing and weight limit specifications, but the following JOF thread may be of interest to you:
http://www.jaycoowners.com/showthrea...H-Sway-Control
Also, check your owners manual for any reference (or restrictions) when using a WDH. As you gather information, please keep in mind that RV dealers tend to speak in terms of dry and/or unload weights, you will want to think in terms of "loaded" weights based on your personal loading habits.
Oh, have your RV dealer provided you with any documentation that came with the WDH, it will be useful to you at a later date when you re-adjust the WDH to your loaded TV/TT conditions.
Bob
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2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (Strait-Line)