Wellcome to the Nissan Forum, it sounds like you have an early case of Buyers Remorse. That can certainly ruin what should be a warm fuzzy feeling that you made the right choice on one of the largest purchases most of us ever make. I am leaving room here for the exceptions, like exotic speed boats, villas on the sea, Ming Vases, or a 100% coverage Family Health Care plan.
When it comes to a new car purchase based on high expectations of great fuel economy, anyone buying a new vehicle right now is setting themselves up for disapointment.
The Auto Makers are all frantically looking for that magic mpg modification that will get them back in the game and people back in the showrooms.
If you check the Find a Car site devoted to mpg numbers you will find a few vehicles out there that in fact do post better than EPA estimates.
Not by much mind you, but mpg numbers that, when the average is taken stand out from the rest. A few examples are the Honda Civic & Fit, Toyotas' Corolla, and Yaris.
Your Versa 18.4 mpg numbers are indeed hideous and right near the bottom of the Versa posted results on this site, but you didn't mention if those numbers where 95% stop and start city driving, or what percentage of a mix of city and highway.
Even the amazing new VW 2 ltr TDI's that are consitently posting in the 40's on the highway, get knocked back into the mid 20's in stop and start Urban conditions.
If you have waded through all the postings on this "Horrible MPG's" thread,you will find a few reasonable things you can take to the dealership for a check up, but from our experiences with the Gass Guzzer Versas, unless there is something so wrong with your car that shows up on the scan, your Service Writer is as clueless as we are about a Fix.