Hard to advise without seeing the structure inside., here is the rule of thumb.
Best practice is to transfer the weight of the rack mounts to the sides not the roof. Usually the most strength is at the roof curve or right at the frame.
The simplest way into mount the rack is with an "artificial" rain gutter, a steel shape that you through bolt. You will see them on a lot of pickup shells, especially the really old ones. Thule, Yakima, all the rack mfgs have them. You are able to use old fashioned "rain gutter' feet and bars, easy to mount and remove to store your tent someplace safe when you aren't using it. Just looked them up, can't believe how expensive they are now -
Thule "artificial" raingutter mounts