Silicone fails when exposed to sun and weather. As others have mentioned, even new silicone will not adhere well to failing silicone. Silicone is a good sealant when used in compression between two surfaces.
If, like the OP, anyone has just applied the silicone around a few windows, etc., I'd suggest to remove it. Use one of the caulk removal products like Mosenbachers or 3M. If you apply the remover and cover it with Saran wrap, let it sit overnight. The next morning, it should peel off easily. Next, use a rag with mineral spirits and remove any remaining film. I'd also follow up with some cleaner and water but if the mineral spirits gets it all, just let it dry.
The best products to use for sealing the windows, doors, and siding on a TT are the urethane or polymer sealants. Proflex RV and Dicor are two of these. I used Geocel 2300 construction sealant (a cousin of Proflex) and all of it is in perfect condition after three years.