CO detectors are susceptible to exposure to long periods of high temperatures, like a closed up rig sitting in the sun in storage. They can also be "poisoned" or give false reading due to some solvent vapors, emissions from furnishings and hair spray. 5 to 7 years is about the norm, but they can go bad quicker. Occasionally, the low battery beeps do not agree with what the label says it should be. The one in our TT went bad after about a year, but Atwood replaced it after I called them. They're the ones that advised removing the detector from the TT if it was going to be shut up for extended periods with no ventilation.
If your detector starts beeping for no apparent reason (you're not cooking or using a propane appliance), take it outside with the batteries still installed. If it continues to go off in the "fresh" air, it's either batteries or exhausted/contaminated sensor. If you install new batteries and it continues to activate outside, it's had it. Never "test" a CO detector by blowing your breath into it or holding it near a vehicle exhaust. That will muck it up for sure.