As folks have mentioned, the fuel economy display is a running average with a "window" of x miles. So yes, a 1000 mile highway trip will get you to where you are more-or-less showing what your highway mileage was since the "window" is undoubtedly lower than 1000 miles in my experience. The accuracy can vary slightly from "real" hard-measured fuel economy, depending on how you drive. I have a light foot and my EVIC mileage has actually been pretty close to "real" based on my spot checks using a calculator and fuel recepts. I'm also showing 17 for my general rural driving right now; slightly higher than it was because with the milder weather, I'm running with the HVAC on manual with just outside air blowing in. Highway, I get about 20-21 manually calculated as well as via the EVIC.
2012 Grand Cherokee Overland Summit 5.7L V8 - Winter Chill with New Saddle Interior