That's a pretty high number. I'm wondering if it got that kinda mileage when it was new. My wife's Nissan minivan barely gets that with a v6. Do a search in this forum, and you'll see a couple of threads where many of the members posted their mileage.
Maybe 20 or a little over on the highway. 18 mixed is really pushing it, unless you have a 5 speed and spend most time on the highway. With an auto don't expect to crack much over 15 around town, even in a good state of tune.
I doubt even a good running 4.9 is going to get better mileage than a good running 5.0 driven the same way. I would believe 18 out of a 4x2 5.0 if it has the right tranny and gears and is driven conservatively with no load. I'd bet that most of you guys that get lousy mileage out of a 5.0 have a lead foot and/or lifts and big tires.