Dick, You know this, but for those who might have forgotten, CO mixes with air mixtures and is at all levels; no stratification, but explosive gasses like Propane are heavier than air and stratify at lower levels. That is why the Propane detector is mounted so low in RV's, and you can mount the CO detector anywhere, but 'head' level is best.
I bet there are thousands of people in cabins in Alaska and homesteaders all over the world with ventless heaters that are happy as a clam.. (and healthier than Clams...)
OH, and the life of a Propane detector is; MAX 10 years, I would change them at 7 years to be safe, and a good idea to test them occasionally.