In a home, CO alarms last about 5-7 years. After that, the sensor wears out or gets contaminated. In the harsher environment of a trailer or RV, your unit may expire sooner. In most newer products, a single chirp means low battery
(or low voltage if it's running on your 12V system) and self-diagnostic errors use various tone patterns - check the manual if you have it, or go to the manufacturer's web site or call the number on the back. If it's emitting a CO alarm when powered up, it's a goner. If you replace the unit, make sure it's listed by UL, ULC or ETL or CSA certified AND is specifically approved for RV use.