When the x-trail is in retarded timing mode (limp) it is revs limited to about 2500 to 3000 revs. There will be no turbo and the retarded timing means you will have no power going up hill even if you floor the accelerator. The gas mileage will be poor as well. As a start I would change the fuel filer on your vehicle in case you have a blocked fuel filter. This is easy to do and only costs a few dollars. You need to wait until your car goes into limp mode and then take your car back to your Nissan dealer. Don't switch the car off as it will loose any fault information stored and get them to run the diagnostic check whilst the vehicle is still running. Your Nissan dealer sounds fairly clueless so I would find yourself a decent auto-electrician/ mechanic to have a look at your vehicle to try and diagnose the fault. As the engine management light is not on it is unlikely to be a serious fault with your vehicle. Problem areas on your x-trail are EGR valve (diesel only), air-flow sensors, MAF sensor, throttle sensors, leaks (various), ecu, turbo, intercooler and stretched cam belts. But it could be almost anything so you need to try and get some clue from the diagnostic system before you start changing parts. If you read some of the back articles on this Forum you can get the car to display a fault code without going to Nissan service department. This involves switching car on and off and pressing the accelerator in sequence. The dash flashes a code and count the flashes to reveal the fault. With good luck and a decent auto-electrician/ mechanic you will track the fault.