They just won't last in the sun.
Both of the RV's stored next to mine in Florida had covers. One was an ADCO and the other was another brand. Both just fell apart after a couple of years. A two-year old child could rip the fabric.
I bought ADCO tire covers. The outside has lasted a year so far, but the cloth like liner deteriorated in a matter of months.
I have had similar experiences with screen tents. The fabric rots in the sun in 6 months.
For RV covers I think the problem is that the fabric has to be at least somewhat breathable so you do not come back to a RV covered with black mold. But unfortunately breathable fabric does not hold up.
I store my fiver without a cover based on everyone else's experience that I have talked to. I do put the reflector cardboard (roll of insulation from Home Depot) in the windows. I am not sure if that does much, but everyone else uses it, so monkey see, monkey do. It sure does make things dark inside.