I had 7+ year old tires on my last bus that I had down to about 30% tread left when I parked it in the Mohave desert for a year then went out on the road again.
It took me about 200 miles to blow 2 inside duals. I limped into the nearest truck tire shop and spent $2k+ on six new Yokohama's.
I completed the trip putting about 1800 miles on the tires. I then parked the bus in the desert. I drove it about 20 miles once a month to try and keep it in shape. After about 4 years the tires were SHOT. Less than 4000 miles on them and the sidewalls had cracks that I could stick a penny in and it would stay.
IMHO: Don't store your bus at the gates of Hell (Bullhead Arizona) and keep your tires covered when stored. Most of all... Drive it!
Tire covers are great, but you should also use a good tire/rubber protectant. I use "Ice 2" . Just spray it on a clean dry tire and walk away. Don't use Bleach White to clean tires. That stuff dries the oils out of rubber.
you can use UV protectant on tires too... i have used 303 aerospace protectant on tires for classic cars parked outside.. it will last through a few rains if you arent driving the bus... im not sure if any of the longer-lasting tire specific products do UV protection or not...