I had the same problem recently but with a slightly different window design. I left the old gasket intact and laid some butyl tape (which has tar in it) slightly on the gasket and to within a 1/4" of the outside of the frame, after seriously cleaning. Then a 1/4" inch bead of Henry's roofing tar along that outside edge next to the butyl tape . Get someone to help you set it back in so you don't slobber tar all over the exterior, wear gloves, the tar will ooze out when you tighten and is a chore to clean up. It may be easier to wait a few days to let the tar harden a little and then cut it off rather than trying to wipe it off when it's fresh. In either case it's a chore but guarantees a good seal. Using mineral spirits, goof-off or wd40 on any cutting or cleaning tools helps.On other leaks, on the side of the rv, I was able to just lay down butyl putty or else just jam it behind the frame with a thin flathead screwdriver, dipping it in water so the putty doesn't stick to the screwdriver.
Good Luck,
Roger