I went from a 4Runner (v8) toa BMW X5 in part because it was a smaller engine and got EPA 21 MPG HWY. On the 4Runner, EPA was 19, just like the GX, however, I frequently got 21.5 MPG Avg on the HWY.
I have since dumped the X5 because the BEST i ever got on that was 15.1 MPG Avg (and a host of other reasons). I now have a GX that i'm very happy with.
Even though they are the same engine, I thought the GX would get worse mileage than the 4Runner due to it's size and height. But's it's all deceptive and they seem to get the same exact mpg. +20mpg on the hwy for a v8 isn't all that shabby.
I towed with my 4Runner and got the same results as towing anything else described here. that's life with towing and a v8 gas engine.
