Bill,
I'm going to put answers and comments in line. I don't like this, but
it will be clearer alone the way.

Bill wrote:
I'm changing my thoughts on epoxying the entire bottom, what I'm
leaning toward now is using Git-Rot to harden up the questionable
frame parts then shooting some under the frame members where each
screw comes through from the outside.
OK
Even if Git-Rot doesn't harden rock hard I'm thinking that it will
flow/seep in and around the screw shank and the hole in the aluminum
skin and block up any water path.
Don't count on that. Git-Rot can not be counted on to hold screws or
fill if it has to go up.
Then I'm going to use expanded foam to fill in the empty area between
the frame and the skin where I removed the water damaged panel.
No foam that you can buy at a big box or building supply will have
anything like the structural rigidity you need to to stabilize the skin
at road speed wind load. You can get high density Divicel(sp?) or
Aerex(sp?) foam that would work, but it costs more than marine plywood.
I'm going to replace the missing pieces that the rot completely
destroyed and sister some new frame members along other questionable
ribs.
Good Plan
Top the entire thing off with a 1/2 inch piece of marine grade plywood
and screw and glue the living heck out of it.
By top it off, do you mean inside the cabover? That still leaves the
skin ineffectually supported. Remember when you were a kid and you put
your hand out the window - Think about that.
=Don't spend the money on marine grade plywood exterior uses that same
laminating process, but may(will) have voids in the core. You can cut
around those.
The 1/2 ply should structurally strengthen the frame, maybe not as good
as having a continuous panel on top and bottom but close and the foam
fill will stop the skin from coming up off the screw heads and opening
up holes.
Let me get this right....
Are you proposing to rebuild the frames, put plywood inside and then use
an expand-in-place foam between the skin and the plywood?
It that is the case, please listen. Without proper controls (a heavy
plywood form) in place, the foam will push the skin all out of shape.
Arrange things so screws go into wood only and be ready re-tighten them
after the moisture content of the wood settles down.
I'm going to try the epoxy/foam on a small area first to see
how it works and if I run into problems your method or a variation of
it will have to do.
Always a good idea to do a small trial before committing resources.

Keep in mind I've never done any glass work,
This would be a god time to learn, but start with some experiments
outside the RV and where you can see and work.
how strong would a sectioned panel be if I just glassed the topside
of it right over the frame down to the panel??
To paraphase:
You have rebuilt the frame and put in a plywood panel and want to wrap
glass/epoxy over the frame to the panel both sides of the frame?
= West and System 3 epoxy are very capable materials. The larger
bending load on the panel will be between the frames, not at the frames.
This could be a very strong assembly, but it may be difficult to
manage. The screws for the skin will have to be long enough to reach
the frame. If you put the panel joints between the frames and are
afraid of the bending load, the use epoxy to do a single fishplate (a 4"
wide - panel high) of the same plywood. When you bond that with epoxy
(you can hold it in place with a few 5/8 dry wall screws) it will be at
least as stiff all the way across the joint as the plywood itself.
I could fill in the panel pieces like you mentioned then basically
lay a fiberglass layer over the entire frame and panels making them
basically one piece??
That would be very effective.
Just looking for options.
Thinking is the cheapest thing to do.... keep it up.
I like having different options so I can weigh the pros and cons of
each before making my decision.
As I said....Keep it up, you get there.

I looked at the westwood and system three products and you're right
that they would do what I want but I'd need about 2 gal plus
WHAT? Remember, you're not going to fill large volumes with epoxy
anymore, I thought we crossed that one off. Even if you did, West epoxy
with microlight filler is only going to be 10/15% epoxy by volume.
- Trust me on that one.
to do it the way I was planning and that adds up quick and I'm on the
tightest budget imaginable, think negative numbers,
Think, you got that right so far - keep it up.
also some people on the RV groups have mentioned worries about the
epoxy over stiffening the overhang to the point that it might cause
problems down the road.
The Gougeon Bros. formulated the West System (West is Wood Epoxy
Saturation Technique) so they could build strong boats. Let those
people worry, do the job right and don't worry about doing it again.
Flexibility in an RV shell is not a positive attribute. How may 30+yo
Airstream, Avion, Travco and GMC (real GMC, not just the chassis) RVs do
you see out there still? The answer is lots... they just don't look old
and they are still in use. They are not flexible. If it is flexible,
it will probably leak. There aren't many caulking systems that can stand
very much movement.

Back to the tight budget issue. We already left out the in-place foam,
and traded off the marine plywood for exterior (just plan on throwing
some away). You can use 5/8 or 3/4 (hard to get) dry wall screws for
everything inside. If you go the fit panel pieces with fishplate
joints, you can do that with a urethane construction adhesive (this will
be adequately strong, but you will have to use more and be sure it oozes
out of any joint you assemble). This would be a fraction of the cost of
epoxy, but it will take somewhat longer (not good in my case).

If you get the whole thing stable and want to squirt in foam for
insulation (sound and heat) go ahead.

My office has recently been upgraded, it was in a 1978 19' Cayo/Dodge.
It is now in a '73 23' GMC. The cabover space was difficult for an old
man to use for tool storage, so I gave up and commandeered the family
coach. Everything is a compromise.

My clients count on me to fix things once - just once. I stand behind
that (some as much a 15 years now. If I were you, I would be worried
about this guy by now. Want to know more? www.southpointechandler.com

I'm at the point in my life that I would like to sell of the hard won
lessons - if I could get 5c on the $, I could live the rest of my life
in comfort. As that is problematic, I just don't want it to be lost
when I check out.

Good Luck Guy,
If I can help more, I'm here a lot.

Matt Colie