The squeak maybe nothing to worry about. Does your closet wall show evidence of moving when you step on the floor or when you support the weight on the king pin vs not? No movement then you have an annoyance. If you pull up the carpet and exposing the fasteners holding the subflooring to the subframe, you can see if the fasteners came loose and add one or two more.
That being said there are two frames. The main frame that most of the fifth wheel is attached to and a front subframe screwed to the main frame. In order to inspect it, you may have to remove the side trim of the exterior overhang and then the screws that hold the fiberglass on. If you have a shroud around the king pin, then that has to be removed. This will expose the two frames and what is on top. From what I have read and my experance is that the frames maybe coming loose and the flooring is moving as the two frames move to and from each other with weight as it was in my case and a few others. The problem is referred to as frame flex. You may not have the problem. Just a squeak. It should not take you more than a couple of hours to inspect what is going on if your subfloor is screwed down tight. Use a blanket to rest the fiberglass sheeting on and inspect the frame fasteners and flooring. I had the fasteners heads shear off attaching the two frames together causing the outside trim to pull away from the side when the fifth wheel was supported by the king pin. Others have had the bedroom floor and closet moving when weight was on top of the floor. Exposing it to see what it is going on is quick and relatively easy if you have the room and working space. I do not, but have seen it done and it isn't all that hard. Depending on what is found, the cure may or may not be easy as the inspection.
Others please chime in if they know of a easier way to inspect the subfloor attachment to the front subframe and the subframe to the main frame..