I once saw a system of how they fitted a swappable fifth wheel assembly on a truck. With some kind of click it in system, ready in a few minutes. Can not find the video back
Anyway, my concept for how this could work, after inspecting the truck more carefully, is following
Ideally, you need a system to raise and lower the truck about 8 - 10 inches. With air suspension, this would be a piece of cake. Since we don't have that, we will have to make four hydraulic legs that can easily attach to the truck or the dump bed. For the moment, we can also do that with two 10-ton hydraulic jacks under the truck.
Dump bed removal
1. Raise the WHOLE TRUCK about 8 inches (jacks, hydraulic legs, whatever I can figure out).
2. In front of the dump bed, there is an option for locking the front of the bed –left and right side– to the sub-frame. Lock that one.
3. Install support legs under the dump bed (drawings will follow later).
two legs will go under the tail of the dump bed.
two legs will go left and right front under the front of the dump bed.
4. Loosen and remove the six bolts on each side holding the sub-frame onto the main frame.
5. Loosen the link between the PTO and the hydraulic pump for the dump body (have to figure a quick way for that, maybe an electric pump would be the way to go)
6. Loosen the link between the operating lever in the cabin and the hydraulic pump. Electric pump with remote or wire controls would solve both problems.
7. Lower the truck. The legs will keep the dump bed high, and the truck can drive from underneath it. The subframe is hanging under the dump bed.
8. Reattaching the dump bed: do all in reverse order.
Fifth wheel assembly.
Quick and dirty: only a fifth wheel assembly, better would be a platform including the fifth wheel and base plate, plates to cover the frame space between the spare tire area and the fifth wheel, and the sliding ramps at the back.
As to fifth wheel, I now saw how they are attached to the frame (see pictures). No need to bother with a separate side rail bolted to the main frame. Just drill four or six holes directly into the top rail of the main frame and ready.
Basically the switching system could be the same as with the dump bed, but much lighter and hence easier. Just make a simple support leg system that holds the assembly high, drive the truck underneath it, and raise the truck.
You further need to add the electric connections and air connections for the trailer. This you can easily do as permanent fix, next to the spare tire. Just make a T connection on the air tanks, one to the back of the truck for your normal trailer, one to the spare wheel area for your semi.
I have to change the lighting anyway for the DoT requirements. I will install a new wiring for the lights and blinkers, so I can keep the original lights and switches functional as well. Quite easy to branch the wiring going to the back with a connection to the spare wheel area as well.
Easy on paper, now putting it in practice.