Tailgate problems of the 2016 Nissan Titan

Our 2016 Nissan Titan xd was idling on a level carport with the parking brake engaged. No one was in the cab of the truck. I was unloading tools from the back of the truck. After about 5-7 minutes, while I was standing in the bed of the truck, the engine revved and the truck began to roll very slowly backwards. It rolled over a small concrete bump (intended to prevent cars from rolling off the carport), down about a 6 inch step down, across another 10 feet or so of concrete and into the brick wall at the back of the carport. I did not have time to safely jump down, open the truck door and engage the brake. The total distance the truck rolled was less than 10 yards. The tailgate of the truck was damaged badly when it hit the brick wall, but it helped to stop the truck. When I got into the truck to drive it back up onto the parking area of the carport, the parking brake was still engaged. I disengaged it to shift into drive. We have had issues in the past with the engine revving and extremely hard shifting between gears. There is also a heavy smell of exhaust inside the cab of the truck very often. The smell is strong enough to cause headaches. We usually have to crack the window when this happens. It happens intermittently, but often. I am not sure if what happened today was a brake problem, engine problem or transmission problem, but this is an extremely concerning safety issue.