Phillip, you need a good quality cover and it will last years. I bought one from Overton's who use a company in Arizona or least used too. They are custom made to each boat and trailer for that matter. They called me to ask what my trailer was and if it had bow stops that the cover needed to fit around, along with if I wanted the swim platform mounted or removed with the cover on. I was very happy with the fit. My Sunbrella cover lasted 4 years outside, a couple more inside and looked great when I sold the boat. Clicky
Personally I would not use 303 on my cover, as I would want it to breathe which prevents mildew buildup.