I have been using 7 to 8 years as a rule of thumb for tires that are used at least occasionally... Holding up the full weight of a motor home and not moving for many years is not a good treatment of the casings...and as you note exposure to UV is another aging factor (but so is just exposure to the atmosphere -- and the ozone from high traffic areas on highways and such make things a bit worse.)
This bulletin from Michelin says to inspect tires each year after they reach 5 years old and unconditionally replace them at 10 years max.
https://www.michelintruck.com/assets...e_RV_Tires.pdf
Chuck