The offroad types like the vehicle mounted rooftop tents because you get comfort of a nice bed pad without hauling it out and setting it up. (And typically without hauling a teardrop trailer which can be challenging for very rough trails!) Most rooftop tents are flip open setup which is faster and easier than on the ground tents. They also keep you away from on the ground nasty creepy crawly, slithery things that might want to sleep with you. My only problem with them is up and down the ladder at night while half asleep and having to pee in the middle of the night. I have a hard enough time getting out of the teardrop like that without doing a face plant.
For the posted problem....
I would unscrew the fantastic fan plastic rooftop cover and remove it. They are very easily removable with a couple of screws for replacement if they crack and break. This will release the pin that keeps the fantastic fan from running with the cover closed. The cover is to prevent rain from entering the teardrop, and the rooftop tent will do that anyway. As long as there is an inch or two between the bottom of the tent platform and the lip of the fan frame, you will get good ventilation and be able to operate the fan. (I would be happy with two inches clear.)
When the rooftop tent is removed, reinstall the fantastic vent cover.
I would check into what the bottom platform of the tent is like. If it is not stiff enough, I would build a stiff platform for it to ride on so that it does not sag down when loaded with rambunctious teenagers. Good luck!
For anyone more interested in rooftop tents, here is a link to some very nice, but expensive ones!
http://www.autohomeus.com/rooftop/maggiolinaAirlander.php