I am towing a 19 Roo with a 2011 Highlander and a Reese hitch...
Pro Series Pro Series RB2 Weight Distributing Hitch (1,200 lb) 49570
The Toyota doesn't seam to even be working that hard. I can stay in OD at 65mph (2,100 rpm) on the level areas and the tranny drops into 4th on slight hills. Two weekends ago going to Cape Cod at 102F
on the cars thermometer and very muggy with the A/C on we went along just fine. The temp gauge never moved and I was watching it very closely.
The Roo actually weighs 3,668# (advertised at #3,170) empty with no battery and empty propane tanks per the sticker on the side and verified at our town weigh station. Loaded (without water) we are at #4,280.
The Highlander gets 12-13mpg on regular at an actual 65 by my GPS.
We are coming from towing with a Honda Pilot which was out of its league. The Pilot always felt like it was working it ass off as shown by always being one gear lower than the Highlander and getting 9 mpg. I consider the Toyota to be 50% better in all respects as a towing vehicle than the Honda.
We considered going to a Roo 21SS this year (advertised at #3,972) but I think anything heavier would just be too much for the Highlander.
Hope this helps. Happy camping.