Info is for my Siesta Sprinter, your construction SHOULD be similar.
Light fixtures, my covers unsnap exposing squarehead screws to remove. My wires are run inside the tubular aluminum framing. (holes were rough and jagged, I smoothed the holes and put plastic loom on the wires to protect the insulation for all the lights.)
Roof: Not hollow- Normally solid stryrofoam with luan plywood sandwiched on top and inside under the ceiling fabric. Air Conditioning ducts are channels molded into the styrofoam. Difficult to chase wires through the roof or the metal channels. Inside ceiling is normally not removable, glued to the styrofoam. May be removable in the front cap area (above the bunk) but easy to damage in the process. flimsy.
Vents are usually prewired, at least for 12volt and often for 120. Remove the vent trim (4 screws) and the wiring should be in the vent frame. Start with the bathroom vent to see how it's done.
Hope this helps, hopefully the wiring is there.
U.S. Coast Guard retired- 1956-1985
2016 Siesta 24ST
1972 Moto Guzzi Eldorado