What CO monitor do you have? Mine is a Safe-T-Alert 45-741 CO/Propane alarm. I would download the owners manual and see what they they say about troubleshooting. Possible alarm causes discussed in the links below and include monitor malfunction, end of life, dust/dirt over cover, assuming you do not have a CO due to unburned gas.
https://www.theoutdoorhacker.com/why...0be%20replaced
https://thecampingadvisor.com/rv-car...eps-going-off/
https://beginrv.com/rv-carbon-monoxide-detector/