SNERK......that Dickson is always a dollar short and a day late.

About 10 years ago when I was planning to drive my 95 Plymouth Voyager mini van up to Quebec to buy my Sandpiper 565 sailboat, I had a transmission cooler installed because of the dubious reputation of Chrysler transmissions at that time.
Brother in law offered me use of his Dodge Ram pickup so I took advantage of that.
Seems to me that the RAV4 came with some kind of cooler and I had no problem towing 33 percent above it's rated capacity....well maybe climbing long hills where my speed would drop 20 mph below posted speed.

I would look into a cooler for the Highlander if I upgraded to a heavier boat upwards of 3000 towing weight even though it's rated for 5000 lbs.
Better safe than sorry.