Chirp sound

Depending on what model you have thy might be in slightly different locations.

The smoke detector hangs from the ceiling, ours inconveniently mounted right above the microwave/convection oven.  Runs on a 9V battery and will chirp when the battery is weak.  The cover swings off to one side where you can replace the battery. 

The carbon monoxide (CO) sensor is on the wall near the ceiling above the bed.  It also runs on a 9V battery that should chirp if the battery is weak or the sensor is going bad.  They have a date stamp on them and should be replaced every 5 years, mine went bad last year as it's a 2010 model.  The whole sensor twists off from a bracket that secures it to the wall and there is a battery compartment on the back where you can remove and replace the battery.

The third sensor is a propane sensor, which is hardwired and runs off the campers big 12V battery, usually on the dinette seat about 8 inches to a foot off the ground, looks like a gray square box with a green LED on the front of it.  Ours has never chirped but alarms loud and steady.  I've found that sometimes stinky sulfur water fumes from a campsite can trip it.  It's only gone off twice and both times we had the gas tank shut off.