If you just put a bead of silicone caulk around the window, there is a good chance that the silicone will fail and the window will continue to leak, perhaps into the frame of the TT, unnoticed. Silicone is a good sealant under compression but is lousy when sealing a butt joint exposed to weather and motion.
Since you already have the butyl tape, remove the window and do it right. Follow up after the window is installed with a bead of sealant around the top and two sides. I would use one of the Geocel Tripolymer sealants like Proflex RV.