Have you ever used one before? If you know what you’re getting yourself into, then great. But they’re not for everyone. Example: if you need to drive into town for any reason you have to tear down your tent and set it up again. You need to park on flat land to sleep comfortably in the tent. Not all camping parking spots are level.
I’ve traveled thousands of miles with a RTT, but I pull mine on a trailer, so I don’t worry about those issues. To be honest, they’re all mostly the same and likely come from the same factory in China. Pick the size you want from any reputable company and go with it. I bet the weight and aerodynamics would KILL our vehicle range.
there might be one for the C40 but the topic of rooftop tents is easily researched on the web. They are truly dumb things. The only reason to be up in the air is to avoid ground-based predators, not many of those around here. The hassle of climbing and possible risk of falling off the ladder gets old quickly. A trailer or a simple tent are both far more practical for camping.
for everyday camping, a ground tent is the most practical. I off road though and pull my trailer with a truck and not the XC40 so weight isn’t a huge issue for me. I wouldn’t consider regularly pulling one with the XC/C40. Though the towing weight limit is around 2000lbs, it’s maximum cargo capacity is only 960 lbs. Pulling weight does not factor in GCVWR.
3 adults: ~525lbs
10% of trailer for tongue weight: 200lbs
Camping equipment: 100lbs
Food and water: 50lbs
Figure with three adults you’re almost at your max and it’s not a good idea to travel far at your max. Can it be done. Sure. Should it be done? Debatable.
At the end of the day, would I get an RTT for the XC? Nope. The reduction in mileage and lack of practicality kills it for me here.