I take it it's like mine the translucent fanlight on two pivot arms/handles. Remove the screws on the pivots note position of the arms then pull them off the pivot collars. Note these pivot arms will only fit in one position because of the stops on the collars.
Either take a measurment of the inside of the frame or if you still have the fanlight take that to the caravan shop for a new cover but make sure you're getting the operating arms with it. Some have introduced lock clips but you don't have to use them.
Replacement reversal of removal You might think it's wrong but sometime you can have problems re-fitting the pivot arms on their lugs persevere you will get there. I found it easier to re-fit the pivots in the open position.
nobby
October 2008