I have a '16 21DK and pull it with a '13 Explorer. I live in Northwest Ohio where it is flat as a pancake. The Explorer is a little bigger and heavier than the Highlander but has a 5000 tow and 500 tongue weight. I believe the torque is higher as well. My setup and experience follows:
I'm within all max specs of the Explorer with passengers, gear, etc, with the exception of the tongue weight. I literally don't have any extra to spare except maybe 200 lbs if that. I'm well over the max tongue weight for the Explorer since the 21dk is over 600 lbs. I've got the tow package, trailer brake, anti-sway stability, trailer/tow assist, weight distribution hitch, and installed airbags in the springs for additional stability. With the WD hitch the end sags 3/4th of an inch from the front when the camper is attached. Since the WD hitch redistributes the weight off the end, I'm not THAT worried. I know I will get flamed for that! Be nice people!
Knowing that I'm maxed out with the Explorer, I have a feeling your capacities like GCWR, horsepower, torque, etc in the Highlander are slightly lower than the Explorer. But, the Explorer is more than adequate to pull it in flat land no problem. In fact we drove it all packed to the max to Yellowstone and back last year for a total of 4,000 miles and it pulled it up all those mountains and back no problem. Now I wasn't winning any speed races in some spots that's for sure, but it did the job for probably our one and only trip up mountains that big. Some areas were 3rd gear and floored and gulping gas so quick I could watch the gauge drop before my eyes but it did it. Not ideal I know, but just trying to give you a frame of reference for a similar sized and equipped vehicle.
I agree with Dcheatwood that a lot of factors need to be looked at and evaluated. I can't wholeheartedly recommend pulling it with a Highlander or Explorer, but if its relatively flat where you are, and you camp fairly local than its probably doable, but obviously not a perfect setup. I'm happy with mine and no complaints (with the exception that I would prefer a real truck) expecialy since it made to YS and back.