I have never pulled a window in an RV but I have done some extensive repair on old wooden boats year ago. I would cut the caulk around the window with a razor and remove it being very careful to not damage the frame. Inspect the plywood inside the frame area and if you have an inspection camera you could thread it down between the inside and outside wall to look for mold or rot. If you dont have one pick one up on Amozon that connects to your laptop USB port for $15-$20. If you have a sheltered place leave a fan blowing in the cavity to dry it out. If you see rot down inside take it to a pro. If you just have some bad wood near the frame get a product called Git-Rot. You inject it into the wood and it hardens stronger than the original wood and is impervious to water. Of course reinstalling the original window is the easiest option but you could have a fibreglass man patch the hole but expect a bill in the thousands for color matching gel coat and forming the fibreglass to look seamless.
