Smoke Detectors and CO2 Detectors actually have a shelf life of only 5-10 years depending on the manufacturer.
When mine was constantly chirping on our previous RV I replaced it and in the instructions, it stated the shelf life was only 5 years!
Just one of the things we don't find in the owner's manuals.
The CO2 Detector recently went on our 2014 Southwind and Fleetwood's Techs (in Decatur) said the original round model had been replaced by a new, rectangular version.
I suspect the original models were not up to Fleetwood's standards.