The CO2/Propane alarms are usually near the door and mounted in or around the fireplace cabinet near the floor. They can be either black or white plastic. They typically have a service life of about 5 years and they must be replaced. They can't be serviced.
Mine went out on the road. Went into an rv dealer and purchased one, it was different brand, and mounted it in place of the old one. Had to mount it a little different, added a couple of holes for the mounting screws, but it works fine. Took me about 20 minutes.Dave