
You may have to look in an RV store to find it, but there is a product called Protect All. It can be used on vinyl, fiberglass, metal or rubber. It is designed to protect to outside of travel trailers/motorhomes. It blocks the UV rays which cause the damage to the finish, or in your case the seats. If you have a Camping World near you they will have it. It's about $30.00 a gallon, but that should last several years. They may also have smaller containers.
I use it on my 5th wheel trailer which is stored outdoors, it works great to protect it. I also use it on my bass boat. Coating all the surfaces, except the carpet, the boat is over ten years old and still looks great.