Sounds about right for a gas engine.
As others have said, it can be all over the place. Pulling the 5th wheel around 60mph in our signature:
- Our old 7.3 diesel averaged just shy of 13mpg long term with a high of 16mpg (strong tail wind and lost 3000ft of altitude that day) but 9mpg one day with a 30mph head wind.
- Our newer V10, is only running about 9mpg but we don't have as many miles.
In hilly country, manually working the throttle can be helpful. Pick up some extra speed near the bottom of a hill and let it slowly bleed off going up the next hill helps avoid a lot of shifting and you don't have to floor it to get up short hills.