Never used the covers when I was actively RV'ng since I parked out of the sun (3-wall carport), so FWIW:
- Lost 3 out of 4 tires on an ATV wagon due to sunchecking / dry rot even in Alaska. It only rolled about 2x per month between May - September and sat for the rest of the time (except for reposition to get out of the way for snowplowing). Lost the tires after about 4 years of that pattern...cheapo ATV type tires, FWIW.
- Several colleagues fly bush planes with high dollar (think $2,XXX per pair) tires on them. They always cover (even in Alaska, and even in the winter) when parked and most will take fabric covers (even though it adds a few pounds) when they fly into the backcountry and plan on the plane being parked for a few days.
- My airplane (with the above high dollar tires) sits in a hangar most of the time, but I take tire covers along when I fly out of town for more than 1 night. My 2-touchdown per landing skills will likely be what dooms my tires, but putting the covers on or off is worth it for whatever protection is provided.