for what its worth, "the fast lane truck" on youtube tested a new lincoln navigator with the 3.5 ecoboost towing a trailer up the mountains in colorado. IIRC the average over 7 miles was around 4 mpg. Worst case scenario but it does highlight that even a relatively small engine can be extremely thirsty when you have a couple turbochargers.
3.6 mpg
https://youtu.be/Q3fjr1G60G8