For RV window frames, the best sealant usually depends on what part of the window you are sealing, but in most cases a good RV-grade butyl tape under the frame and a compatible non-sag sealant on the outside edges gives the most reliable result. If you are resealing the window assembly itself, many RV owners have the best luck with butyl tape because it stays flexible, seals well when compressed, and makes later removal much easier than hard adhesives. That matters on RVs, because windows often need to come back out someday for repairs, and a sealant that bonds too aggressively can turn a simple job into a major headache.
For the exterior finishing bead, look for a sealant made specifically for RVs that stays flexible and is compatible with aluminum, fiberglass, and the window trim material you have. Dicor non-sag lap sealant is often used around vertical surfaces, while self-leveling versions are better for horizontal roof areas, not window frames. Around windows, self-leveling products can run and look messy, so they are usually the wrong choice unless the manufacturer specifically recommends them for that spot. A non-sag polyurethane or RV window sealant is generally a better fit for vertical seams and frame edges.
I would avoid regular household silicone unless the window maker specifically allows it. Silicone can be difficult to remove later, may not adhere well to all RV surfaces, and can make future resealing harder because other products often will not stick to silicone residue. Once silicone gets on a surface, cleanup has to be thorough or the next sealant may fail early. That is why a lot of RV techs prefer butyl tape plus an RV-rated exterior sealant instead of going straight to silicone.
The real key is surface prep. Old cracked sealant should be removed as carefully as possible, the area should be cleaned with an approved cleaner, and the surface needs to be completely dry before resealing. If the frame is loose or the gasket is worn out, sealant alone may not solve the problem. In that case, tightening the window, replacing the gasket, or reinstalling it with fresh butyl tape may do far more good than smearing more caulk over the outside.
If you want the longest-lasting result, I’d lean toward butyl tape for the mounting seal and a quality RV-grade non-sag sealant for the perimeter. That combination is common for a reason: it seals well, stays flexible, and is still serviceable later. META: Best sealant for RV window frames explained, including butyl tape, non-sag RV sealants, and what to avoid for a lasting leak-free repair. ETIKETLER: RV windows, sealant, leak repair
