The lower half of the Centurion box is only about 5 1/5" deep so retrofitting it with the PD would not work, the PD is deeper than the Centurion box. There is a thread here somewhere where someone cut the back off the Centurion box and enlarged it to hold a PD converter and kept the Centurion breakers and fuses. That brought it real close to the tub and requires having some metalworking skills.
It isn't all that hard to remove the CS3000. As I remember, each cable has a clamp on the back of the CS box that is a bit of a challenge to release so you can push the cable out and pull the CS box forward to remove it. I pulled one cable out on mine when I added a wall switch for the water heater and fortunatly there was nobody around to hear some of my choice words as I did that.
One problem getting my CS box partly out was the solid, heavy ground wire from the converter to the trailer frame. It went out the back of the box and through a terminal strip. All the DC negative connections also went to the terminal strip and I had to loosen that copper ground to allow the Centurion box to pull out part way. I had to access it from a second panel in the bathroom under the tub.
Maybe some super quiet 12 volt fans or an on/off switch for the fans would be easier. I have seen some super quiet fans (26dba) online for $10 to $20.