Just ponder a thought :) If we wanted "good gas mileage" we would not buy a house on wheels with poor aerodynamics and drive it down the road at 65 mph.
Knowing the mpg and size of your fuel tank just gives you and idea of how often you need to stop for fuel. Example: 10 mpg x 39 gal tank, I stop every 300 miles (give or take). "Saving money" is what you saved on the hotel bill or by eating the food in your fridge vs eating out.
Enjoy the family and the trip and don't worry about fuel mileage :)