I don't have a Winnie so what I'm about to tell you may not apply exactly to your situation.
I have a 1977 Midas Mini and we have rebuilt about 1/2 of the cabover bed area (we removed the bed from ours). I can tell you how it is put together and a little of what we had to do to fix it. Yours will probably be similar. And we took ours apart from the inside.
Just so you know... we have found that our framework is all stapled together and only on one side... It's like they laid out the frame (exterior side up) on a floor and then stapled it together.
Starting at the bottom of the cabover, directly above the cab's cutout ceiling..
A layer of aluminum which extends out to become the exterior bottom of the cabover/sleeper
1 inch thick plywood... ours was rotted from the bottom side (pinholes in the aluminum). I cut out the bad wood with a small router that uses dremel-type tools (set slightly shallower than the thickness of the plywood to prevent cutting the aluminum). I glued pieces of aluminum flashing (thin enough to cut with scissors) over the holes. I then pieced in new plywood to match the thickness of the original wood. Then glued & screwed 3/4 inch thick plywood over all the original plywood. In order to get the piece in, we had to cut it in half widthwise.
Originally there was a short piece of plywood on each side of the cabover that extended from just under the bed to the back edge of the cabover (ours had a bed that had a trundle section to pull out to become wider). The trundle section of the bed was plywood that "ran" on a plastic track. Our plastic track was severely damaged and was falling apart. We had no problem removing the bed mostly because we had removed a wall cabinet on one side of the camper (otherwise we have needed to cut the bed in half).
The sidewall framing (2x2 inch) extends below the plywood. The exterior siding is glued (contact cement) to the framework and the 2" thick sheet foam insulation. The windows are framed out with the same dimensional lumber.
The front section (ours has a window in it... it had leaked and rotted out some of the framing there) is framed out like the sides.
The framework of the cabover had a lot of rot in it. We removed the rotted sections, cutting back far enough to hit good wood. And then carefully fitted replacement pieces. We also made sure there were no more leaks. Exterior corner trim was removed, cleaned and replaced old butyl tape with new. Replaced old screws with new standing seam metal roofing screws, sealed edges of reinstalled corner trim with (white) Henry's brand elastometric roofing sealent to prevent dirt/water from causing butyl tape to fail. All light were removed, replaced, resealed with elastometric sealent. Replaced all the paneling with new paneling. Result is a very solid cabover that does not leak.
You need to post over on the Vintage forum. While we all have different makes, they tend to be constructed pretty much the same way (all the ones from the 70's seem to have the same fuse box!
). And we all have run into the same or very similar problems.