The reason an exhaust exits out the rear of a vehicle is so that Carbon Monoxide Poisoning doesn't happen from exhaust gasses entering the truck.
Carbon Monoxide attaches to the hemoglobin 200 times more readily than oxygen. But then the red blood cell is nearly unable to release the carbon monoxide and the red blood cell "dies" with the carbon monoxide still attached. When enough of these red blood cells have attached to carbon monoxide you go to sleep.
BUT THIS SLEEP IS USUALLY FOREVER. aka you are dead.
Going from memory: in an atmosphere of less than 1% after several hours you never wake up.
It just isn't worth the risk of death by staying near your exhaust which spews out carbon monoxide. It is odorless and tasteless so you will have no warning of imminent danger. Keep the exhaust near the end of any vehicle.