You need to contact the maker of your camper. They should be able to answer your questions along with on how much weight you could put on top which you really need to know.
Or, use a stud finder to find the roof raffters if any. As far as what size screws, I would look at any place were there is a preopen like a roof AC or bathroom roof vent. That should give you an idea on how thick the roof is. Making any holes thru a rubber roof, you would put down putty tape, then the track, screw it down. Trim most of thye putty that squeeze out without cutting thed rubber roof. Then apply some Dicor sealer on top of each screw head and around them and on each side of the full length of the track.