Not to sound critical but you have to realize that you're working against two factors: lots of horsepower and heavier vehicles. You're going to get less gas mileage than a smaller vehicle would get no matter how you go about it. So, IMO, I would just go with more horsepower and get the most out of it. Make it a beast and get a Towncar for regular driving.
I had my 06 Silverado tuned by @06MonteSS and have loved the extra power and even with playing with it more I have noticed a 1-2 mpg improvement. I'm sure if I stopped getting on it so much it would go up another 1-2. Well worth the $480 in my opinion since going from getting 16-17 mpg around town to 19 is nice. I haven't had any long highway drives to see the improvement there yet.
dont buy cheap gas. only mobile, shell, and stuff. never arco or valaro
upgrade your ignition coil as high as you can
get NGK spark plugs
use B12 chemtool in the tank, vaccum lines, and trottle body ever 25,000 miles