While your boat sits out in the sun so does your trailer tires. Does it help to cover your tires to keep the sun off them and if so what do you cover them with?

Yes sun is bad for them.

We made some covers out of plywood for our TT tires since it used to be stored outside. The boat has always been garaged.

RV supply stores sell them ready-made, cheaper than you can make them(unless you use garbage bags).

I had one on mine forever but I dumped it. I have chrome rims and water collected at the bottom of the wheel and caused rust. Maybe I should have gotten a better cover. Then one I had was made by the guy that made my cover.

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.