Re: highway MPG
"how long is a piece of string"
It will depend on so many variables: how much was the FJ modified? (lift, larger tires, roof rack mounted tent/box/lights, and so on), how much load is being carried? (some have hundreds of lbs of gear/refridgerator/air pump/fuel/etc.), what altitude? what grade of fuel being used? AT vs MT?
As you said, you already know what the FJ is rated for, MPG. So, if yours is unmodified, and getting 17mpg, the fuel tank will read empty with about 4 gallons left (15~16 out of its 19 capacity), you'll be wanting to stop for fuel at around 250~300 miles, which will be about 3.5hours at 80mph.