Well personally I would NOT tap into the power source for a saftey item like fire/propane/c02 detectors. Dont sound right to me and im sure in a residental home it would be illegal.
I would look for a light or a switch that is close by or run a seperate wire from the fuse panel over to it. But that just me.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor