I would find a replacement bed, I found one from Texas for mine that was rust free. My bed wouldn't pass MD inspection due to rusted supports and I looked into replacing the supports but opted to just get a new bed. Check FB marketplace for people selling truck beds. I would get a new (used) one as yours sounds like its done for. While you have the bed off the truck you can work on the frame and clean it up and seal the frame to prevent further rust damage.
Here is a good thread on it:
https://www.ford-trucks.com/forums/1440853-rusted-longbed-support-sills.htmlI wouldn't use it with the camper on till you fix the issue. The good thing about our trucks is the beds can be changed from many different years (paint might even match), I put an 06 bed on my 02 truck. Or just turn it into a flatbed.