The problem is this:

You can buy a new, more fuel-efficient SUV that can seat 6+, but they all cost around $30K or more (Honda Pilot, Chevy Tahoe Hybrid, Toyota Highlander Hybrid, etc.). The amount of money you save from gas is not going to make up for the amount above the mid-$20K you are willing to spend, unless you keep the car for a loooong time.

It might be more financially feasible to keep that Navigator until you no longer need such a big car (i.e. once your kids all grow up and flew the coop), then trade down to a more fuel-efficient, smaller car.

EDITED TO ADD REPLY TO C7S:

Dude, she is asking for an SUV that seats 6+. The Saturn Vue is a compact SUV/CUV and does not have 3rd-row seating.