We just returned from a 1091 mile trip including climbing the Grapevine both northbound and southbound. I have a '09 Tacoma with the V-6 engine and automatic trans (TRD-Sport) and we were towing a total load (trailer and cargo) of 4000lbs with about 300lbs in the bed of the truck. Our average fuel mileage was 14.63 mpg for the entire trip. Our worst mileage was 13mpg (northbound from San Diego to Delano) heavy traffic, steep mountain and heavy rain. The best mileage was 15.59 from Delano CA to Elk Grove CA (flat central valley farm land.)
Overall, I'm pleased with the mileage. It is a little better than I was getting with our F-150 equipped with the 5.4L V-8. Our Tacoma handled the load fine and once I adjusted the hitch. lowering the front of the trailer 2-inches or so, the ride and handling was much better.
PS: I kept the trans in "D" when ever I could, dropped into 4th on any uphill pull. Also kept the speed around 60-62 (just below 2000-rpm.)