A vent cover is very much worthwhile even if it doesn't solve your problem. Being able to leave the vent(s) open all the time if you desire is a fabulous thing, and very helpful particularly for storage. Having one in the back open while driving also helps pull air from the dash air conditioning or heat through the whole RV.
I think your theory is sound; these gas detectors are sensitive to things other than just CO (or propane), and certainly sticky aerosols could clog them over time. If you need to replace the CO detector again, perhaps it would be simpler to get a battery powered household detector and stick it up wherever is convenient and appropriate. It would be wise to use the test button before traveling, of course, but that's really true in any case.