Yep the uv will break down the tire rubber over time. Dry rot sets in. I use them all the time even carry them on trips since most all our trips are a week or longer.
I just bought new tires for our trailer, the original ones (only 3 years old) were very badly cracked, but they were the most cheapest tire cruiser rv could find and they were the Duro.
I bougt 4 tire covers off kijiji (same as craigslist) for $5.00 a piece I would like to save my tires a bit now.
IF you read thru any of the various RV/motorhome information postings, guidelines and forums about tires from long time RV'ers ~ nearly all recommend checking the tire mfg date on the tire.