When driving slow there are a multitude of things that will creak. Leaf springs with plastic bushings that are worn out. Hitch to rails in the bed. And the chin area.
If it is the chin...it will be a bit more complicated than removing the closet for a fix. The way to check chin/pin box deflection is like this. Tape/secure a tape measure to the front cap that hangs down to the pin box. Write down the measurement with the 5er unhooked from the truck. Then hook up and take a new measurement. If the difference is more than 3/4" or so...there is your problem.
Knowing what I do about how cheaply 5th wheel suspensions are built...I would look there! I have rebuilt mine twice in 4 years and about 35k miles.