Somewhere around NOV/DEC 2011 the vents changed for the stove exhaust. The old style has tabs that rotated to secure the vent open or closed. The new style, you have to push up on the tabs pretty hard or insert a flat tip screwdriver just under the tab and twist. Now closing is a different story. It is a good thing I am 6"2" tall and not vertically challenged as some have admitted to.
Another thing, when you use the stove vent and cooking on the stove, you have to slightly open the window next to the stove. The todays RVs are so air tight, the vent can not exhaust the heat from the oven or stop cooking, by the window being open there is a supply of air for the fan to move the heat from the stove and a fresh supply of oxygen.