I've gotten as high as 14 (flat land near the gulf coast) and as low as 9 (florida pan handle is hillier than you would think, spent a lot of time in 4th gear). Without towing I get about 16, maybe 18 if I baby it.
It's going to vary based on your driving style, maximum speed, acceleration rate, willingness to be passed by snails, etc, etc. I experimented with 87 vs 93 octane and didn't see much difference, but didn't go the same route.. at most I would expect 1 mpg difference or so.
They say the liberty normally gets 22 on the highway... if that's true you could expect high teens. If it's an automatic (don't know much about towing with automatics..) you might turn off over drive, or put it in tow mode, especially if you're in a hilly area.
1995 Toyota 4Runner SR5 4WD 5 spd
2002 Eagle Summit 12