An old engine and sheer mass work against the 2014 Lincoln Navigator, and it's only rated at 16 mpg combined (14 mpg city, 20 mpg highway) in rear-wheel-drive models. Those equipped with four-wheel drive do worse, with ratings of 15 mpg combined (13 mpg city, 18 mpg highway). It's officially a flex-fuel vehicle, with the ability to run on E85 if you happen to be near the 1 percent of U.S. gas stations that offer it--but then fuel efficiency falls to 12 mpg or 11 mpg combined, respectively.
However, if you fill each of the seven seats with a passenger, fill the cargo hold with stuff, and decide to tow a boat, you'll find that the environmental impact per person isn't all that bad. With a luxurious space for up to seven, plus cargo--with some towing ability built in--it just might replace two vehicles on a road trip.