Hi you will have to put in 4 battens around the edge of the hole you cut in the roof.
Pity it isnt frosty because when it is you can see where all (the few) roof spars are with the frost.
Just be mindfull of cables going to the lights.
I knew someone who did the same thing a few years ago but in order to avoid an expensive mistake he contacted the relevant caravan manufacturer for advice. They kindly sent him a blue print of the roof structure which illustrated the batten positions and dimensions so he could fit the corresponding roof vent without any interference with the main structure. He did manage to conduct the task successfully which was a complete success along with a professional finish so it may be wise to take the same steps and contact the manufacturer for advice first.