I've searched other RV forums for peoples experience with a 4Runner. Most agree the 109" WB is a limiting factor for trailer length and the 4.7L V8 has plenty of power to pull most trailers up to 23' and longer. I have not read of any issues with the hitch failing or torquing due to weight distribution or weight not being transferred correctly from a WDH. Most of the posts I read involved stick trailers and mileage figures ran from 8mpg to about 12mpg. I would think towing an Airstream would be a better towing experience vs the stick trailer.
Here is a quote from a Bigfoot owner, August 2011 with a 2004 Toyota 4.7L V8. Mine is a 2005 4.7L. Toyota bumped up the horsepower at bit in 2005 from 235hp to 270hp but torque is similar 330ft lbs at 3400rpm.
"I just towed 1500
miles from Austin to Ruidoso, NM pulling a Bigfoot 22ft 5000lbs travel trailer. I've logged approx 10,000+ miles using the Runner as my tug vehicle and it does a great job! I never had to floor the truck, even while climbing the Davis Mountains during this trip. The MPG avg'd 10.1. However, I keep a fairly constant speed of 72MPH on I-10 (the speed limit is 80) and actually got the truck up to 90MPH down a long, rolling hill. I have near zero sway and it only squats in the back approx 1 inch or so, but I use the Equal-i-zer 4 point sway and weight distro set up. I doubt I would pull anything without this hitch. It makes all of the difference in the world. I use the Prodigy brake system which I also like. "