I recently came back from a month long road trip which involved towing my travel trailer. A total of 5774 miles were driven on the trip. I would say that 90% of those miles were driven while towing.
The trailer I towed was 18ft and about 5000lb when loaded up. I also had about 600lb of payload in the Tundra.
I used an app called GasTracker+ by Fuelly to track my MPG. Two thousand six hundred eighty seven (2687) miles were tracked with the app. This was so because if the Tundra was driven even for a second with the trailer unhitched, those miles were not counted.
Seven point nine (7.9) was the worst and 11.2 was the best MPG during my trip. The average turned about to be 9.4MPG.
My road trip consisted of driving in high altitude and/or up steep grades as well as flat land. That is probably what accounted for the range in MPGs.
What are you towing with your Tundra and what are your MPG numbers?
3rd Gen Toyota Tundra 