Also, is your exhaust coming out straight out from the bumper or out the side?? If it is exiting straight out, it is in the low pressure area between the TV and trailer. Like said above, it could be entering the cabin vents at the rear of the vehicle. Running fresh air into the cabin will create a high pressure area in the cabin. I would look into relocating the exhaust exit point to the side of the vehicle to get it out and into the air stream going down the side of the vehicle.