diagram of aluminum construction

I don't think what you want exists.

If you write to the manufacturer with your year, model and VIN number, they may have specs available.

Have you searched YouTube and Google or other browsers to see if a photo or video has been posted showing the construction. Some RVers tour the factory and may have posted photos or videos.

My 5th wheel (now defunct manufacturer) developed several stress cracks over the years. The first one appeared at the top of the 5th wheel above where there is a large window. The crack (now 1/8" wide) has travelled down to the window frame, about 18 inches long. I have filled it with flexible silicone seal and painted it.
The other cracks are hairline and short.

It may depend on the sub-brand of FR product; Wildcat, Wildwood, Rockwood, etc.
I suggest you Google "2006 Sales Brochure" for your particular brand.

This is probably about as close as you'll come- this one's a Forest River Cedar Creek model:

This time of the year will not work because of the heat but during cooler months I can see the framing when the weather is cold on mine.

I have a crack and wrote the co and they replied "it's out of warrenty, go see a dealer. They may be able to help you" how's that for being blown off. End of story It's cracked and has not gotten larger.

I asked Forest River(I have a Wildcat) for the possibility of adding another window in the back. They sent me a diagram of the aluminum frame construction showing me where the frame was.