I have been at Alumapoluza this weekend, I replied to your email today however it bounced back.
We have several customers with the same Highlander that you own as well as a large number with the Lexus version and Sienna's which use the same drive train. Most are towing 25-30' Airstreams and some quite extensively with trips to Alaska etc. That I know of all are quite pleased with them and we have had several move from older to newer ones.
The 19 or 22 are both easy for the Highlander. There will be no issues with handling stability etc. On some long grades you will shift down and climb in lower gears at about 3800 RPM but it will run on the level at 60 MPH in the 2300 RPM range. A large V8 or diesel will climb the hills 10 MPH faster but they will never save you enough time on hills to make up for the extra time you would spend in gas stations.
With either the 19 or 22 you absolutely must use a weight distribution hitch that is adjusted properly. I would suggest an Eaz-Lift brand with 600 lb. bars and a Husky friction sway control. There are no issues with the body structure, today unit body vehicles are actually much stronger than body on frame vehicles.
The Hitch that comes on the 08 Highlanders is not terrible but also not overly solid. Do you have the Factory trailer hitch or do you need to add one?
The picture below is a 1999 Lexus RX300 it was the first Toyota crossover we set up. This one had a 3.0 Ltre with a 5 speed transmission instead of the 3.5 and 6 speed that you have. It towed the 25 Safari to Alaska, Panama, New Foundland and several trips to California.
"One test is worth a thousand expert opinions."
Tex Johnston, Boeing 707 test pilot