Welcome to the forum!
Let's get this out of the way first, because it's going to come up!
Why not drive something more efficient to begin with? With the amount of money (fuel) the Excursion will burn in a year, you could pay for another vehicle (an efficient beater), and every year after that it's money in your pocket, plus less wear & tear on the big Ford for when it's needed.
(Or.. why drive the Excursion in town when you have a Focus?)
I realize that this may not be an option for you, or you'd rather just work with what you've got for other reasons. But it's worth asking if you've sat down and run the numbers.
Getting a gauge was definitely the right first step, regardless of what you drive.