It would take a close inspection of the RV in person to have any certainty in answering your question.
I think it might not be a practical modification for a few reasons, though. First, and perhaps most significantly, I suspect putting a king size bed in that side of the bedroom might block the closet doors from opening, and would almost certainly make it very tight to walk around and access anything.
Second, there's a quite good chance that the slide mechanism is contained within the bed itself, and is not underneath the floor.
Third, but perhaps the least concerning, there may be some difficulty involved in piecing together the floor such that the sliding part is narrower than it originally was.
Personally, I don't care for the design of this particular bunk arrangement. It looks very hard to access and make, with the only entrance being at the foot (or, I suppose, the head) of the bed. However, that's hardly relevant to your decision, and I'm sure most young children would be thrilled with the arrangement even if the people taking care of them think otherwise.