I've had 4 travel trailer, a 93, a 94, a 98 and a 2003. All the vents were the same. Take out 4 screws to remove the trim ring. Then you take out 2 screws PLUS the screw out of the crank handle. The screen and crank handle will now come off. This is all done from the inside of the trailer. I know this because twice a season if not three times my wife has me take the vent screens out so she can clean them.
As I think about it maybe your MAXAIR vent covers might be interfering with the vent not opening all the way.