I've seen a roof reinforcement for bike racks listed as a feature or option for some models, e.g. in 2013 it was a feature for the Jay Series but not available for the Sport.
So for pop-up roofs that can handle the racks, there is likely a specific place to mount them, so that you fasten to the reinforcement plates. Locations should be in owner's manual or dealer's service manual for units that have this feature.
Otherwise, you will be liking to find the roof framing, if any. Other places will be just roof skin with air behind, or at best foam insulation.
The roof is often thin enough that lacking reinforcement, you could install your own, sandwiching a section of the roof between two reinforcement plates, one inside and one outside, and installing the racks with bolts through the plates.
You prevent leaks by using sealants appropriate to what you are mounting. At the minimum, putty tape under what you mount, and you can improve with lap sealants over and around.
Tom Test
Itasca Spirit 29B