I've never understood this - a gasser, with the engine in front is quiet in the back but there's no alarm. My DP, with it's clacking, pounding engine would wake the dead when backing up, yet some manufactures have added an alarm. Just another RV industry mystery.
I just went through all of my fuses and mapped where they were at. Not one is marked for the back up alarm. If no one answers with a location, you can do as some have and just add a switch in the engine bay or just add an inline fuse by the alarm.
I would love to have a switch on the dash, but haven't been motivated enough to run another wire back there.