A full-size pickup is about the last thing that comes to my mind when I think "good gas mileage." But then there aren't really any vehicles in your price range that can comfortably seat six, also fit their luggage, handle towing and light off-roading, and get good gas mileage. A large pickup might best do all of this as long as you can sell two people on sitting in the middle.
If not for the off-roading requirement I'd suggest a Toyota Sienna minivan with all-wheel-drive.
Given this requirement, maybe a Honda Pilot. It has more room for luggage than most semi-offroad-capable vehicles its size thanks to a deep well behind the third-row seat. I'm less sure the third-row will have enough space--depends on the size of your people. Fuel economy should be a few mpg better than with a large pickup. By "off roading," do you mean unpaved roads, or more than that? The Pilot should be okay for rough roads, but probably not for venturing where there is no road at all.