Well, the 12v comes from the batteries, of course! Just kidding. Sorry.
Have you lost the needed 12v?
FYI, my LP detector would alarm when refilling my LP tank....and would scare the cat something fierce. So I put a small switch on the 12v wire so that I kill the power to the alarm under these circumstances.
Robi & Dale ---- Go Navy ----
2016 Winnie TOUR 42'
Tow: '08 Buick Lucerne