The trickY part is how the the cab and bed would be divided. If it was one large open area I wouldn’t think the stock AC and heater could manage all that space. Not to mention there would have to be some way to seal the gap between the bed and cab. It would be nice as you could potentially fit 2 adults and two kids or even make a bunk bed in the bed below.
Above, I made an attempt to describe how I would handle the "sealing the gap" issue. If I were Ursa Minor, I'd make a custom cab hard top that was mostly similar to the stock hard top but with a cutout in the rear. Then also make a separate "shell" that attached to the Gladiator bed with a cabover portion and a matching cutout. Then I would use one of those flexible rubber "accordion boots" to seal between the cab and the shell. That would allow the bed and the cab to flex independently but you could still have a pass through into the cab.
The pic below is how someone else managed the flex with their setup. As you can see, their flexible boot is secured to both the cab and the camper so the two can flex independently. This is a Gladiator though and not a Ram 5500 so if Ursa Minor only made a new fiberglass top with a cutout, you would still need to step over the steel front of the bed/rear of the cab. I sketched in a couple red lines where I imagine the bed would be and a blue line where I'm imagining the roll bar would be.
Accordion Boot Rubber Material Replacement Order by Feet
Order Accordion Boot Replacement Rubber material to seal truck cab to Campers, Toppers and Audio Punchthrough. Sold by feet
www.accordionboot.com