The framing varies from brand to brand. Usually there'd be a wood or metal ring, at least, and hopefully tied into the rest of the cabover structure (and not just floating, surrounded by styrofoam). Water damage would tend to occur first to the luan skin around the opening, inside and/or outside. If the framework is aluminum, it's obviously not going to be affected by water; and neither is the styrofoam core of the wall. You're also somewhat likely to have damage to the bottom part of the front of the cabover, where the bunk board meets the front end, and whatever structure is there since gravity will make the water collect there until it eventually finds a way out.
Removing the window unit is indeed pretty straightforward, but rather awkward without help and scaffolding. You take out the plethora of screws that hold the compression ring to the window frame, take the ring away, and pull the window unit out, possibly after trimming off any caulking or other sealant holding it in place.
Personally, this is a job that I would take to a repair shop if there's a decent one around as they'd likely have the help and the staging ready at hand. I similarly am happy to pay other people to do exhaust system work on my vehicles when needed, even though I could do it myself, as the money involved seems a bargain compared to the frustration of rusted together bolts and junk falling in my face as I scrunch around on my back.
