Just completed a long trip from Ontario to Washington State, and now up to British Columbia. Carefully monitored each day's tow mileage (the truck's built-in mileage readouts are very accurate when compared to average hand calculated values).
Drive up to 62 mph (100km/h) max, use regular gas and cruise, and only very rarely stomp on it.
The worst mileage was in Eastern Montana fighting headwinds: 9.8 mpgUS (24.0L/100km).
The best mileage was going downhill from the Continental Divide, Roger's Pass MT to Spokane WA: 13.6 mpgUS (17.3L/100km).
Have done even better on the Florida flats with tailwinds helping out.
So wind and aerodynamics is a really important factor on the flatlands.
But my light weight trailer definitely helps in the mountains. We're looking at heavier trailers. Total rig weight will increase by 25-40%, and I'm sure the mountain mileage will take a similar hit.