This is a pain in the You know where.
I've had the skinny little hinge pins break on the flap of my last
2 trailers.
The manufacturer has to know they are prone to break.
It would be SO EASY for them to slightly modify the mold that makes
these and beef up the size of the pins but they don't do it.....
Luckily I found my vent hanging by one corner and didn't loose it.
IF you find your flap broken but not missing, you can easily repair it.
I fixed mine last week after a trip in less than an hour and just one
beer!
IF you've still got your broken flap.
First remove the 2 visible screws at the lower left and right corners.
Then take a box knife and carefully slit the caulk around the edges
of the outer vent.
Carefully pry up the outer vent.
I used little brass jewelry box hinges from a hardware store.
They were 3/4 by 1 inch and cost $3 for a 4 pack.
Once the outer vent was raised up it was simple to add the 2 hinges.
I also found large gaps around the duct inside the wall.
IF the vent flap ever stuck open it would allow wasps to get into the
wall cavity around the vent so I caulked the gaps.
These gaps are visible once you open the flap.
Sorry, I didn't take a photo of the unit with the flap open or missing to
show the gaps underneath.
I used 1/8" aluminum pop rivets.
Note the closeup of the hinge where I used a sharp box knife to notch
the top edge of the flap so the hinge fit exactly in line with the upper
edge.
Note-- I removed all the screws and then discovered I didn't need to
do that. Just the 2 outer screws are necessary to make this repair.
Click a photo for a larger look.
Good luck!