Most detectors have 2 hot leads so that they can be powered from 2 different 12v sources in the RV. Typically 1 hot will run to the house battery side, and the other to the chassis battery side. The intent is to make sure the unit is still powered in case you disconnect one or the other (house or chassis batteries). I have seen several that use a common ground (only 1 ground wire) that will work for either + source. To verify just use your volt meter or test light to either hot to that single ground - you should see 12v from either, provided of course your disconnects are not turned off.
94-Newmar Kountry Star 40-KSDP
Spartan/Cummins 8.3C-300HP/Allison 3060 WTEC-II/25yr RV Tech RVIA Certified/Onan-Cummins Certified