Transmission Slip Out Of Gear problems of the 2013 Nissan Frontier

Transmission won't go into gear forward or reverse at times. When driving on the road transmission won't shift out of 3rd gear into over drive, after topping a hill and going back downhill. Truck will do a jerking motion when pulling out from a stop. On interstate i81 truck speed was 50-55 mph the rpm's was 6,000 and the rpm's would not shift into higher gear. When it did shift it was a very hard shift. It has been going on since the day I bought this truck brand new. I have had it to the dealership numerous times and they can not find out what it is. The truck doesn't through a code, so they can't work on it till it does. I pull a small pop up camper at times and it acts up more when the truck is under stress. I have where the Nissan dealership has seen the truck and their comments. They did offer me an extended warranty till 2020. I don't feel safe driving the truck on the road and especially when my family in the truck. I only have a little over 23,000 miles on the truck as to date. Nissan denied my request for another truck because they said my claim falls outside the time period for filing claims. I have the repair paper work and letters from the Nissan group.