I think one thing to consider is what your plans for your TT are.
If you are going to keep it 15 years than you probably should make every effort to keep it nice and shiny.
I believe the average trade in time for a RV is 6 years and like any other vehicle the trade in value drops like a stone in water. Even with the best of care older TTs look “dated” because of the style changes of the newer models.
Our TT was three years old when we bought it and judging from the wear and tear on the cover. it had been under cover much of the time. I am sure the like new condition brought the dealer a few extra sales dollars, I am equally sure the great condition did not add a nickel to the trade-in value.
I use the cover because it keeps the ice and snow off the roof. I do not use it in the summer.