Covers don't have to be used. As mentioned above, you can instead just spray the tires with 303 Protectant.
However, the 303 Protectant should be reapplied monthly, where the tire covers would need no attention (unless someone steals them) between the times you use the RV regardless of how many weeks in between.
I use 303 Protectant on our RV's tires - but it's parked out in the back yard when not in use, so it's easy to reapply it periodically.
2005 E450 Itasca 24V Class C