The proper way to reseal the window is with butyl tape under the outer flange when it's (re)installed. It's sealed to the outer skin of the RV. I would not try wrapping eternabond around the opening for the window as the extra thickness would likely cause difficulty with the window installation and sealing and doesn't really seem to me to help the situation at all. Aluminum foil tape, such as is used for ductwork, would in my opinion be better than eternabond for this application partly because it is thinner, but I'd still go with no tape at all.
If there's a bit of a gap for access, it might be possible to squirt some adhesive (epoxy possibly) down between the outer skin and the plywood or whatever behind it to reattach it (and then rig up some sort of bracing to hold it in place while it cures) prior to resealing, but it hardly seems to me to be necessary for such small areas. Of course, it's not always easy to judge how much damage there is before actually looking at things.
