Thanks for the replies.
Reviewing the specs for the 2018 xle awd, it has a heavy-duty radiator with engine oil cooler, 200-watt fan coupling, supplemental transmission oil cooler and 150-amp alternator (that's from https://www.toyota.com/highlander/features/mileage_estimates/6942/6950/6953).
I've been through the manual several times and it basically says stay within the weight ratings and be careful when towing. It recommends using a sway control device for trailers with a gross weight above 2000lbs.
According to the manual the v6 Highlanders meet the SAE J2807 standard for towing trailers up to 5000lbs. From what I've read the SAE J28807 tests assume a driver and passenger at 150lbs each along with 70lbs of hitch equipment.
Reading an article at Edmunds (https://www.edmunds.com/car-reviews/features/no-go-for-standardized-tow-ratings.html):
"Toyota was the first to begin certifying vehicles to the new standard beginning in the 2011 model year. It went all-in, too. Every model in the Toyota lineup has been rated according to J2807 since then."
I know the Highlander is designed primarily as a passenger vehicle and towing a 5000lb gvwr trailer seems unreasonable to me, but a 3500lb gvwr trailer seems doable as long as I watch tongue weight.
