Thanks for the reply, Big Blue. I've never had a fuel system service/flush performed. I don't recall seeing that in either schedule A or B maintenance in the owners manual. I suppose it couldn't hurt; I doubt it's ever been done. I'm still curious what others are seeing as far as MPG. 12.2 just seems really low, to me.
As I said above, the plugs were changed when the transmission fluid/filters were changed. I used the NGK Iridium IX spark plugs as recommended here on the forum. The Champion plugs were extremely worn so I suspect they're the original.
I thought only the Hemi's had 16 plugs! Man, that's a job! Anyway, I was hoping for a smoking gun answer, but that'd be too easy. Seems like it could be a 100 different things causing this problem.
As far as your gas mileage goes, I've had mine dip down that low when I do alot of stop and go driving.
It's also good to reset your MPG calculator every so often so your MPG readings aren't being bogged down with old data.
When I'm on the highway I get around 19 MPG, with the lift, bigger tires and all the extra weight I've added to it with all the upgrades I've done, that's not too bad.
I recently added a cat-back exhaust & cold air intake thinking it would help my mileage but that doesn't seem to be the case.
It did give me at least 20 more hp though, I can definitely feel and hear the difference.
As far as pinning down your issue to one thing, you just can't do it - unless you happen to go to the dealership and maybe they get lucky running a computer diagnostic and find something or if they check your Commander's computer and come up with a trouble code where they can pin it down to something specific.
Other than that, you can only eliminate the potential causes by going through them one at a time.