Your discription of the bubbling and separating paneling reminds me of the way I found the leak in the very first motorhome we owned. Same symptoms that the interior paneling separated.
What I did was removed the window completely by first removing the interior metal trim then removing the window from the outside. Once the window was removed, I chisled out the softened 2x2 underneath the window. I actually didn't have to remove the entire 2x2 but only about 1/2 the length of it. Since the 2x2 was rotten, it actually removed pretty easily once the window was out.
I next ripped to width a replacement pice of lumber to fit within the wall cavity. The lumber was installed using gorilla glue on the ends to join up with the rest of the framing as well as the edges to bond to the exterior and interior wall finishes. Since the window was out, it was easily to clamp the interior and exterior walls to the new lumber and allow the glue to cure. The gorilla glue expands slightly as it cures and fills any voids or gaps betweent the lumber and framing so the replacement lumber was solid. I'd recommend using backing lumber, perhaps a couple 2x4's on the inside and outside while clamping the wall section as the glue can expand and will spread the wall if not evenly clamped.
After curing overnight, remove the clamps and backing lumber an dyour frame will be solid. After the repair is complete, reinstall the window using fresh butyl tape and your leak will be eliminated.
You are better off replacing the wood as you'll not get the window to stop leaking if the metal frame of the window can't clamp to the motorhome wall securely. It's the clamping action that causes the butyl tape to seal to the motorhome exterior wall.
Good luck with the repair.
Joe & Shelly, Justin, Tyler, Alyssa | Butler PA
2008 Tiffin Allegro Bus 43QRP|Cummins 425|Honda CRV