Hi lukenick!
It may sound strange but you have to start from the back and work towards the front (so to speak). Before we talk trailers, let's talk cars.
First, please look at your owners manual and give us the tow limits of the vehicle. There's usually a chart that says with 2 people it can tow 3500 pounds (if properly equipped with tranny cooler, etc) or with 4 people 3000#, etc. If not properly equipped with a tow package, the number is lower.
The Sienna is a minivan and they are generally limited to around 3500# tops. I have a popup that I towed with a Chrysler Town & Country minivan that had the tranny cooler and just about wore it out towing the popup (it has a slide out which adds weight). Then I upgraded to a truck (Suburban) which has been wonderful.
The point is we've been down this road before with posters and your vehicle is the limiting factor (next to your bank account of course). People try to rationalize too heavy a trailer and it's just not safe for you and any loved ones travelling with you.
The issue is not being able to tow a trailer; the issue is being able to
safely stop and control a trailer in traffic.
So do this - look at the max towing limit of your vehicle: example = 3500#. Use 80% as the max safe towing limit of this which yields 2800#. Trailers have several numbers/weights associated with them. Compare the 2800# to the Gross Vehicle Weight Rating (GVWR) of the trailer. This is the max weight the trailer can handle (empty weight plus cargo/luggage). If the GVWR is greater than 2800#, it's not safe.
Others may have a different safety percentage other than 80%. But the principle is the same. You'll probably find that most travel trailers have a GVWR greater than a minivan tow capacity. Minivans are good for popups, Aliners, etc. Don't let a trailer salesman lie to you. Your insurance company probably won't support you if your trailer is overweight for your vehicle should something happen.
<they know how to do the math>
A. Tell us the tow numbers in your minivan owners manual. (do you have a tow pkg/transmission cooler?)
B. Tell us how many people will be in the van going camping.
C. Tell us what type of travel trailers you're looking at (hopefully small and light. Find their GVWR).
We want you to enjoy camping and arrive alive.
Bama Ed
PS - we have lots of smart campers and drivers and haulers who I'm sure have their own opinions (even if they contradict me which is fine - if I'm wrong then I'm wrong; I learn something new everyday). I hope they weigh in to help you too.