I towed with a '08 Highlander for a while, it did OK with it's 5000 lb tow rating, but I had the same feeling you mention that I was wearing a nice vehicle out prematurely. A lot of our driving is in the mountains. My gas mileage was around 17 MPG average when towing.
We upgraded to a 2006 Tundra. The mileage isn't as good (14 MPG avg when towing) but there's a lot to be said for the extra capability of load and safety. I enjoy being able to take pretty much anything we want within reason and not have to worry about it. Never being a "truck" person, I was really surprised how much I enjoy driving the Tundra. Smooth, quiet, very comfortable vehicle with a V8 exhoust note. Being between a full size and mid size pickup, it's alright around town too.
Not meaning to be an advertisement for a Tundra, really meaning to just say you may want to consider more than MPG if you have a second vehicle.
