I see many campers that have varying degrees of sag in this area. Most are not rotted or leaking water into this area.
You have to think about the amount of area this usually thinly structured area is together with the constant weight of two people sleeping there.
If this area where to be structured to carry a normal floor load i.e. for two people it would need to be as thick as a house frame 2x4's,6's,8's,etc. The size of the framing thickness takes up head room above so they go as light as poss. on an rv being hauled. The size of normal weight rated framing depends on the amount of spacing between each frame member and the span distance to the load point on the rim.
Anyhow the strongest structures in wood are those that use douglas fir old growth (very expensive)or standard Doug fir with a glued and fastened plywood skin on top and the bottom. I think that most TC manufacturers are using 2x2 framing (very lite but not great for weight carrying)with a lite interior and ext. skin.
In contrast to your experience I bought a brand new camper and have had many leaks. No leaks into the bed area thus far but does have the same bed sag you are showing.
In the first place your camper is too close to the cab roof so add some spacing under the camper in the bed of the truck. Next is to get the leak stopped and open up the bed structure like you say and rebuild it.
Always remember the frame size and spacing with a glued and fastened skin adds lots of strength.
My bed sag is so weak/ flexible you can see people turning in bed from the outside.
