Based on what I have read on the RV Forums the covers only last about 5 years for a good one and a few years for the cheap ones. If in a windy area the cover may only last through one good wind storm.
In addition, most RVers get tired of putting the cover on and taking it off. We have stored our 20 year old 5th wheel in three different RV storage lots. In the lots only 1/3 of the RVs were covered. The bottom line, the majority of RVers don't or have stopped using cloth/Tyvek type covers.
The best way to maintain the exterior of your RV is to wax it once or twice per year and wash it regularly. Our 20 year old that has sat out in the weather 24/7 everyday since new, still looks new each time it is detailed with the exception of the decals (which we removed).