I have no idea what is the best cover to use, it probably really doesn't matter as long as you install it without a lot of movement when the winds pick up.
After all, do the full timers who live in them without covers dry up and delaminate faster than those who cover them ?
If you desire to cover it :
Some people use clothes baskets on the roof to add pitch to the cover as well as adding air space for better air flow.
If you have rain gutters or other sharp edges, you can noodle them so you don't tear the cover (you'll more years of service out of it).
Some people add additional rope tie-downs to limit the movement of the cover against the rv walls etc. A loose fitting cover can wipe the off the decals and strip paint.
But, at the end of 10 or 15 years, all these RV's need to be cleaned and re-caulked through regular maintenance. Im not so sure the covered ones hold up any better. Inside storage might be a different animal.