Has anyone used butyl based caulk rather than butyl tape in window replacement or other applications where water-proofing is crucial?
The main problem with most "caulk" type products is they simply aren't designed to be used with fiberglass or plastic, and don't adhere well to those surfaces. Butyl tape, on the other hand, is massively sticky stuff that has the consistency and adherent qualities of recently chewed chewing gum, and because it doesn't "cure" like caulk products do, it retains its pliable sticky character for many, many years and moves and gives as the fiberglass shells of our campers move and give with each bump in the road. (Standard silicone-based caulk just shears off the surface it's trying to adhere to.)
Butyl tape is a great product, but there are caulk-like "adhesive-sealant" alternatives that are designed for the fiberglass, plastic, and metal products found in boats. Most marine stores carry the 3M Marine Adhesive Sealant products; the "Fast Cure 4200" product should work well for most fiberglass trailer applications as long as you want the thing you're sticking on to be permanently attached; since there are times when you may need to remove your window frames, to replace broken glass, rotting wood or un-bend a jammed aluminum slide, it might not be a good idea to glue them in with 4200. Another thing to consider is that a tube of 4200 costs something like $20 and has to be used within 48 hours of when the seal is broken; I just bought a whole roll of butyl tape at my local RV shop for $7 and it'll be just as usable in five years as it is today.
So here's where I'll use 3M 4200: to permanently attach the plastic hatch frames I just bought into the holes I've cut in my trailer's sides. 4200 is a good choice because, unlike butyl tape, 4200 will bond the frame to the fiberglass, allowing stresses to be borne by both fiberglass shell and the hatch frames, and this'll prevent stress cracking in the fiberglass at the corners of the hatch. There is some prep involved, first I'll have to rough up the fiberglass under the hatch flange with 80 grit sandpaper, then wipe it down with MEK (Methyl-Ethyl Ketone) solvent to remove any dirt or wax on the gelcoat, and I'll have to be really careful when removing excess 4200 that oozes out as I screw the hatch frame down. The stuff can be removed with a utility knife once it's hardned, but it'll leave marks on the gelcoat. Always use nitrile gloves when candling MEK (skin irritant, cancer causing) or 4200 (skin irritant, very hard to clean off skin without using MEK).