Do you want good gas mileage, good range, or both? If you just hate stopping for gas, you don't necessarily need great MPG as long as you have a big tank. My D'max truck was still seeing about 17 MPG with my Four Wheel Camper in the bed on 35"s and the stock 3.73s. With the ~60 gallon tank I have (aftermarket replacement tank) I have a ~1,000 mile range. Now, 17 MPG isn't "great" as far as MPG, but 1,000+ miles IS great for range. We'll have to see how it does now with the 37"s and 4.56s. ;)
Assuming you actually care about MPG more than range, the next question is what do you need out of the vehicle? How do you Overland? Do you soft-road, or do you need something that can do the Rubicon? How many people do you take with you? Do you need a pickup bed, or do you want an SUV? Do you plan to sleep IN, on, or near it? It's hard to recommend something without that data.
If you only soft-road, and don't need a ton of space a Compass Trailhawk might be a good fit. They're rated for 30 MPG highway and are still reasonably capable off road. They only have a 13.5 gallon tank though, so range isn't spectacular. If you need a bit more space and capability, maybe something like the Cherokee Trailhawk that is still good for 24 MPG (and has a 15.8 gallon tank, making range comparable to the Compass TH). If you need still more capability and space you could move up to the outgoing body style (WK2) Grand Cherokee Trailhawk that is 25 MPG in a V6 or 22 in a V8 (though in practice the gap is much smaller than that) and has a 24.6 gallon fuel tank giving a pretty decent 500+ mile range even for the V8. If you want the MOST out-of-the-box off road capability you could even go with a Wrangler Rubicon with the 2.0T and get a claimed 24 MPG freeway along with the 21.4 gallon tank for 500+ mile range, which isn't too bad for something that can do what a Rubicon can. Of course, all of that is just in the Jeep lineup, and isn't even considering the 4xE (mostly because long freeway trips aren't their strongpoint and they don't provide a benefit there).
-TJ