Does your trailer have any other sliding vents or roof vents?
I have a Sundowner and along with the sliding vents at the head, there are some at the bum end and a roof vent for each horse as well. It also has the "jail bars" behind the regular door too so if I need to I can open that, but I tend not to like to have the doors open w/o a screen when moving as gravel, cigarette butts, etc can fly into the horse's face. If I only have 1 horse I will open the door with no horse to give more airflow.
They say money doesn't buy happiness -- well happiness doesn't buy horses!