I put 140,000 towing miles on a 2008 Highlander. I was towing 16' and 17' trailers for work and for recreation, with trailer weights between 2200 and 3500 lbs. It did well at 60-62 mph max speed in 4th gear (out of 5); on long grades I'd shift to 3rd and run mostly about 40-50 mph depending on the steepness. Stability was good (I kept 10%-15% of trailer weight on the tongue). By the time the odometer had 185,000 the transmission light was coming on when traveling those steep grades, and I thought about adding another tranny cooler but in the end I sold it and got a used Lexus GX470.
Keep in mind that the 3.5L V6 will do best with smaller frontal area trailers. My largest frontal area trailer was 7.5' wide by about 100" high.
There are some people on the Escape Trailer forum who tow 19' and even 21' Escapes (molded fiberglass made in BC) with newer Highlanders. I personally feel like 21' is pushing it a bit; one would definitely want an anti-sway hitch!