Tires which are not used on a regular basis will dry rot!
This can happen in as little as 4 years. When used on a regular basis, heat builds up and chemicals within the tire wall work to the surface and help protect the tire. It sounds crazy but you can do some google searches and find more detailed information. The dry rot killed my spare tire stored under the bed of my pickup. After 4 years I put it in use. It looked good initially but cracks formed with just a few hundred miles of driving.
An RV tire cover will not prevent this. It is likely that Aerospace 303 will but it requires thoroughly cleaning the tire and then spraying a good coat on both sides of the tire. If exposed to rain and the elements the tires need to be retreated about every 3 months.