I doubt you'd EVER get 12-13mpg, even unloaded, no toad, and empty tanks! I think when someone asks a salesman "what kind of mileage does it get" that's a trigger for the salesman that mileage is very important, so naturally - like most salesmen - they bump it up a bit, in your case by about 25%
Our salesman only fudged by maybe 10% or so when we bought our Solera - he said 16-17mpg, and we've been getting mostly 13.5-15. Of course, at my ripe old age I've heard all the fibs, and don't believe any of them. I don't know anyone who's been getting 16mpg with their Solera.
That being said, your driving style can impact the mileage greatly. A light load, a light foot, downshifting on hills to stay within the torque curve, maintaining constant speeds if possible - all have a bearing. You'd be surprised at how much weight you can add to a coach with just what we consider "essentials" which aren't really essential! I installed ScanGauge on my Solera, and it really helps in maximizing mileage (and alerting to any problems with engine, etc.).