I have some ideas to build drawers, but need to decide what would be steel, whats wood, etc. My thoughts are in something behind the side panels that you can thread bolts into to hold it all in place and then disassemble when not needed. My previous sleeping platform was just floating and would jump off the floor in certain off-road scenarios.
I've entertained the concept of a raise/lower platform, using linear actuators or RV jacks or something. The idea is drawers at the bottom, then a fixed platform, then a moving platform. With the "bed" up, lots of gear can be stowed between the platforms (and it can't hit you in the head if there's an accident). When you arrive, pull out all the gear and then lower the platform. This effectively uses all the cargo volume while still leaving room to sleep inside.
The hurdle is that a standard mattress takes all of the length available in a K5. Even with the truck tailgate, which saves a couple of inches from a power window tailgate, we flip the front seat backs forward. Now as the platform goes higher, the space available shrinks, to where the bed is getting into your headroom and you can't drive. So the execution would require something to fold or otherwise compress the mattress lengthwise substantially.
If it worked, a cool bonus would be the ability to level the bed inside a vehicle that isn't completely level.