IMO, GCWR is a SWAG number, arrived at in negotiations between Engineering and Marketing. Marketing, of course, wants it as high as possible, while Engineering wants to keep it moderate, so they find a number they both can accept.
I have never been officially weighed; not in the motorhome and not in a pickup or truck while towing or hauling firewood.
I have weighed one rig, a 1994 Dodge 3500 CTD towing a 32 foot fifth wheel. I found that it was UNDER the stated GVWRs and GAWRs, but a ton and a half OVER the truck GCWR (the 1994 Dodge had a ridiculously low GCWR; Dodge increased it in every year after that, without changing the truck!).
That truck did just fine crossing the mountain passes in Montana, Washington, and California with that trailer. Nothing bent, nothing broke, nobody was injured or died.