I have one of those crummy vents in my trailer. It does not move much air. Last year I had to fix the vent so it would close properly. Whoever originally installed the fan put it on backwards so the vent opens up towards the front. I almost lost the lid on a trip when the gear box failed an only one screw still holding it in place.
When you take down the light fixture you will have access to the bottom of the fan/vent but it doesn't do much good.
I did most of the work from on top of the trailer and had to replace the gear box with screws and rivets if I remember right.
If you decide to replace the fan/vent I remember seeing someone at a trailer rally had replaced the original fan/vent with a remote control model so they could retain the light fixture.
Steve, Christy, Anna and Phoebe (Border Collie)
1994 Classic 30'11" Excella - rear twin
2009 Dodge 2500, 6 Speed Auto, CTD, Quad Cab, Short Bed
Hensley Arrow hitch with adjustable stinger
WBCCI # 3072