I have previously covered, in detail the advantages and the "why" we should be storing our tires inside or under our RV or under light colored covers. It is the HEAT that kills tires and the aging effect on tires is exponential with an increase in temperature (doubles each 18°F) you could end up with a tire failure with ZERO miles as seen in this example.
I pointed out, with temperature data, in my blog post on why white tire covers are good and much better than black covers.
This shot shows what can happen with a black cover that effectively places the spare in an "oven" where cooling air can't offset any of the Sun's heat.
These folks do it right.
I know of no tire shine or protectant spray that provided protection from heat.