Locating Internal Aluminum Structure??

I am wanting to mount some roof racks up top and a vertical bike mount on the exterior back wall of my truck camper. The camper is a welded aluminum frame with sandwiched fiberglass, ply, styrofoam, ply, inner cosmetic panel construction.

I need advice on how to find the aluminum square tube structure under the roof membrane and fiberglass exterior walls. I have read that a stud finder can be used. I purchased one of the advanced types that can locate metal studs and interior wiring, and it can, but it appears it can only find steel studs and copper wire, not aluminum.

Any advice is appreciated.