Finally hand calculated the fuel consumption. The last trip was 19.76 liters per 100 km. I haven't weighed the trailer but safely assume at least 4500#. Plus 500# minimum in/on the truck if you include the weight of the tires compared to stock.
The trip before was similar, but that tank had some travel without the trailer on it and came in at 17.48 l/100
For those using freedom units, that's 11.9 US MPG for last trip and 13.45 US MPG for the previous trip with some non towing mixed in.
91 octane canned tune, shifts set to firm, corrected for tire size.