So here is my replacement board. I cut the original in half while it was still in the camper then bisected the rotten half to get it out. I used the good half as my template for one side and the radius. The width is 4' and the length was ~ 79.5" . I have one coat of ACE barn and fence paint on the bottom side. Will add one more coat of paint then coat the exterior and edges of the board in the Duplicolor Bed Armor.
This corner photo shows the aluminum extrusion on the bottom of the bed board and the trim band. I drilled the large rivet out of the trim band rubber strip on the drivers side where the over cab portion of the camper meets the main body. This allowed me to peel out the strip to have access to all of the screws. I removed all of the screws from the lower extrusion and the trim band.
EXCEPT! for the bleeping trim band screws behind my Fiamma Awning. No access to these screws without removing the awning, so I just ground the screw tips off (and all the staples) from the inside. This way they won't interfere with my new board when I place it. Could not find any online instructions on how to remove my awning but did not want to remove all of the screws holding the mounting bracket on.
A view from the inside. Looking at the bed area. After grinding off all of the staples, I have been scraping all of the old caulk off the aluminum extrusion and frame. Getting it good an cleaned up to caulk in the new bed board after I have it in. The two wood boards that get in the way of the bed board replacement can be seen on either side of the frame, they are painted white. Getting the new bed board past these will be the pickle, as you can see the black gap below the boards where the bed board will seat into. Marty from ATC said, "you will earn your beer after getting that bed board in past those boards". I had also missed some screws along the leading edge where you climb from the camper into the loft area above the sliding window. There are about ten screws along that board into the aluminum frame. You need to remove a plastic trim piece from the inside to get to those screws. This plastic trim was siliconed and stapled. Unfortunately it did not survive the dismantling. Hardware store will have something that will work as a replacement.
Gonna be a few days for paint to dry and before I can make the big push to get the board in. I'll provide an update and more photos later in the week when I go for it.