Our '98 Grand Cherokee with the 318 average 14.3 MPG over the 120K miles we put on it. That is combined. On the highway it averaged about 18. This was with the Quadra track full time 4wd. My daughter has a 2001 Grand Cherokee with the 4.0l 6 and averages about 18. But it doesn't have a tow package on it.
We just got a 2004 Liberty with the V6. I will be putting a class 3 hitch on it sometime after Christmas to tow my 13' Columbian up to Canada this summer. Since it is my wifes car and I have only driven it a few times I haven't looked at the trip computer to find out what the gas mileage is.
If I had to choose between a 6cyl Grand Cherokee or the Liberty for towing purposes I would choose the Grand Cherokee every time! The wheelbase on the Liberty is very short making it much less comfortable to drive and putting a trailer behind it will only make matters worse.
Just as an fyi: The 1997 Dakota with 318 and part time 4wd got 14.6 MPG average. My 1993 F250 Diesel 4x4 gets 18 on the highway and 15 average. And got 10 towing the 27' C-Dory back from Texas. The newer 6.0 liter Powerstrokes are supposed to do much better than my big 7.3l non turbo does. But I will say this. The F250 is so much better in the towing department you just can't compare!
Something else to think about. A diesel is good for towing for a couple reasons. First is the awsome torque. Second is that the extra weight of the engine helps keep weight on the Front wheels when braking. This is VERY important. Cars are designed to have about 60% of your braking on the front wheels. If you unload them by adding weight on the hitch you never get that back.
Here is my tip of the week

When you back down a ramp you are further unloading the front wheels making it nearly impossible to stop. To counter act this when backing down a ramp shift into nuetral (or first with the clutch in on a standard trans) as soon as the weight of the boat starts to pull you down the ramp. This way you aren't fighting both the weight of the boat and the torque of the engine. With the car in Nuetral keep one hand on the shift lever in case you need to quickly pull forward (And so you don't forget to put it in drive before trying to pull off the ramp!).
After 5 years of driving a tow truck I learned a few things about what NOT to do while towing something... I had to pull a couple of vehicles out of the water... I also had to tow a couple vehicles that far exceeded the weight limit of the truck

After the last one I told my boss I would never do that again. Having to hit the brakes to get enough weight on the front tires to steer was NOT FUN!