^^ The roof can take the entire weight of the car, as the rails are mounted flush against it (and are resting above the B and C pillars, which are ridiculously robust.
Weight on the rails is not the issue, the crossbars is what has the load limit. They mount to the rails and are mostly plastic. They are rated for 200 lbs. I would go for a robust mounting method such as aftermarket crossbars made of metal.
My factory cross bars loaded with my basket, two bike rails with bikes attached, and some gear in the basket, you could move the basket, and see the strain on the crossbars. I wouldn't mount a roof tent to the factory crossbars, I'd get aftermarket ones.
Another thing to think about too is that all of your combined weight is not on the roof while driving, which would cause stress upon the mounting system when you took turns, stopped, accelerated. The vehicle is stationary so the weight isn't a problem at that point.....unless you put a bouncy castle up there, then there could be an issue.