^ this
Another option for roof rails that I used was low-profile aluminum strut 'rail' or channel - also known as 'UniStrut'. Typically used for mounting electrical conduit or boxes, it's available slotted, unslotted, in galvanized or zinc-coated steel, etc.
I went with the low-pro (13/16" height) aluminum to save weight - but I had to special order them from McMaster-Carr. Even paying exorbitant shipping for four 10 ft sticks, it was less than half what any of the electrical supply houses around town were quoting to order it in for me.
The actual connectors (cone nuts, etc.) are commodity items easily sourced at most hardware stores.
I contacted the RV manufacturer (Adventurer) and got a print showing the layout of the roof structure. I confirmed using a stud finder, and was able to sink screws into almost all the studs - both the aluminum tube framing, and the few wood members. There were a few that I couldn't because of location - I used well-nuts for those.
Pretty sure it's better attached than most setups using z-clips.