If we could put our Elkridge under a hard roof cover, like a carport, I'd do it in a heart beat.
As for soft covers... I think the risk of breaking something (on the RV, or the person doing the work) when you put them on and take them off out-weighs the benefits. The roof should be designed for many years of sun, wind and rain.
Our unit is stored in the Seattle area, so too much sun - especially during the off-season - is not normally a problem.