Keep them coated with a uv tire protectant as well.
A lot of people thinks it's weird to "armor all" trailer tires, but it helps.
I use vasoline on the tires on my utility trailer and pop-up camper. Also boat trailer spare if I remember. It gets a little messy but that is what they make latex gloves for. I quit buying Armor All after the upholstery guy that was making my tonneau cover showed what it does to vinyl, leather, and rubber. He said do not use it on anything that needs to breathe.