I believe that the OEM vent covers are non-UV stable by design. If they were UV stablized they would sell about 1/10 as many as they sell. A milk jug left outside will outlast a new RV OEM vent cover. So yes you need the aftermarket vent covers or stock in the OEM covers.
No man has a good enough memory to be a successful liar. Abraham Lincoln
2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch