The answer, is turns out, is: scrape off the thick layer of sealant slathered* all over the junction of the top of the back corner piece and remove screws. Fool around carefully with the putty knife until you've released the corner piece from the sealant gluing it down to the roof. Lift it up until it's free from the corner molding running down the outside wall corner, then sideways (away from the center of the RV, so rightward for the right corner piece, viewing it from the back). It should come loose from the clips holding it in place. Wiggle and cuss as needed until it's loose.
Now repeat the sealant, screws, release from sealant gluing down routine for the long center piece. Be sure to separate it from the other corner piece. Once that's free, you should be able to wiggle it sideways until it's free from the other corner piece. Now you can move it out far enough to gain access to the connection of cable to camera pigtail.
My local RV parts house says they stock a Sikaflex product that's the correct sealant to replace the stuff I removed.
Thanks for the link to the wiring diagrams. I'd found them and realized that the camera cable is routed around the outside edges of the RV wall-roof juncture, saving me fooling around trying to fine "attic access" in the back room of the RV. That's all fine; it was that last six inches that was making me nuts.
*This may or may not be a factory slather; it's possible that a past owner had a lot of sealant and got carried away.
