Professional looking caulk "gaskets" around windows
There are three caulk-adhesive products I would recommend, having repaired trailers for about 5 years: Colorimetric, Silaprene, and GeoCell RV Pro Flex. I prefer the first two, though the ProFlex is fine also. Use the regular caulk-adhesive, NOT the self-leveling that is made for roof repairs as it "flows" for a long time. If used on anything but a horizontal surface it will flow right off or puddle at the bottom.
Apply like regular caulk for cracks, joints, etc. If using around a bubble window or roof vent, run a thick masking tape (or two layers of masking tape atop each other) around the edge of the item to be caulked. Place one strip about 1/2" farther out from the existing "gasket" of caulk. Place a second strip of masking tape inside the existing "gasket". You should now have two lines of masking tape 1" wider than the existing caulk "gasket" (1/2" on each side).
Apply the caulk-adhesive between the masking taped lines. All of these caulks are petroleum-based, so smooth them with a smooth (softer) paper towel or cloth saturated with mineral spirits or similar solvent. Do not over-work the caulk, just make it reasonably smooth. Allow 10-15 minutes for caulk to set up. Remove the masking tape slowly so as to not disturb your handiwork. If you wait longer the tape may be hard to remove & tear often due to film strength of caulk. Allow the caulk to set for several days without succumbing to the temptation to touch it; this caulk takes a little while to cure but remains flexible for years.
You will discover your hard work has resulted in a professional looking "gasket" that overlaps the original seal and provides additional protection. CAUTION: 1) Apply in a ventilated work area. 2) These are ADHESIVE-caulks, so removing them once they're cured is HARD to do without damaging your trailer! But the quality of this repair is excellent. 3) Clean-up requires petroleum-based solvents. 4) Wear disposable gloves while working with the caulk-adhesive.