And to add to what mfa said, in my experience, many different GS models use the same base wireloom/wiring configuration. It would not surprise me if there was a power stub up there. If not, you might be able to tie into the roof air relay power somehow with an additional switch to toggle between the roof air or the fantastic fan when the thermostat trips...but that last comment is more of a guess...
I guess at this point, you can cut the hole and if there isn't one, get out your fish tape!
2004 Ultra Supreme Class A 30'
- Added hard-wired Pro-Sine 1000 Inverter
- Converted batteries to 4x6V from 2x12V
- Added Bully air-horn
- Added Safe-T-Steer