It is riveted to the ribs, same as the walls. If you look at your wiring diagram the wires come up the walls and across to the lights, vent fans and ac. No wires run the length of the ceiling. What you will find under the aluminum is insulation and you will be blocked at each rib from pushing the wires through.
1994 30' Excella 1000, Chummy III- Ford Excursion- 7.3 Turbo-diesel