I've always purchased a cover for the various RV's I've owned since I've never had any covered storage for them. I've mostly used ADCO covers,and they generally last around 3 or 4 years.
I used a tarp once and it rubbed some of the finish off of my class c I had at the time.
I'm backwards from most,where I don't use the RV in the summer months so it's covered more to protect from the sun than the other elements.
This year I bought a cover that claims to have a lifetime guarantee.
I can't recall for sure, but I believe it is a Perma Pro cover.
It wa a little more expensive, but I believe the rip stop nylon it's made of will help it last longer. I'm sure down the road I will be testing it out to see how well the "lifetime" warranty holds up.
With any cover you need to be sure any sharp edges are covered so not to rip the cover. I've used everything from duct tape,to gardens hose,to hard foam padding,and still seem to manage to rip a cover.
This time I found some plastic downspout covers and the new cover made it through this summer withpout a tear.