My '97 was rated at 15mpg highway when new. I think that's at 55mph on level track, lightly loaded, fairly ideal mechanical and environmental conditions. Mpg was a big concern to Toy when they built it, and they didn't skimp on quality components. Jumping your rig from 12 to 17mpg sounds unlikely or at least difficult and expensive. I've calculated over 16 on long trips, heavily loaded with a big roof carrier. I doubt that was true, but I sure want to believe it. You'll probably get some improvement with tuning, but it's still shaped like a brick. Lowering and adding ground effects will get more mileage per dollar than engine enhancements, but then why own a cruiser?
I lost 10mpg from my Audi to this. It's kind of a chunk of money, but it's worth it! I hit my fifth deer in it, drove through dangerous snowstorms, slept and camped in it, my wife's been rear-ended twice in hers, my family has been kept safe in both. A kid in a suburban was just killed by a hummer here, our odds are better than in a Neon, and I'm paying for it. I have a big bumper for deer, wish I had big tires, but try to stay practical.