Tom, I looked through Atwoods main service manual and I didn't see a vent nor do I think there is one that goes to the outside if that is what you are refering to. Believe it vents heat and whatever the same as a house unit does but the upper plate or ceiling of the oven getting hot and the heat excapes through the grille in the back of the stove top.
Have someone compounding and waxing the roof of the Excel today to can't get into the unit to see it in 'person'.
Are you asking if it is vented to the outside, or if you think it is vented out through the top of the oven. With a hole that big, you would have a hard time building up enough heat in the oven chamber to bake anything and end up heating the unit instead?????
From as clean as the oven looked, would guess the mouse is looking for someplace else to get a free meal. Maybe the Chief's house.