Hi,
It would probably be a lot less expensive to go up there with some tub and tile sealant, and then caulk the upper side of the window. Perhaps the sides as well, but most have special requirements for the lower section, and putting sealant over the "Drain" holes is REALLY bad thing to do, and can cause water to be retained inside of the walls, rotting out the walls and overhead cab structure. SO if you are not sure about the difference between the parts that must be sealed, and the lower drain slots, don't cover both of them.
Fred.