We also have a 2003 24ft Minnie. The bottom of our sewer compartment hasn't rusted out (yet), but if it does in the future I probably won't bother fixing it. The space isn't sealed from the outside anyway, as there is lots of gap around where the pipe from the gray tank comes in. If anything, having the bottom plate missing will make it easier to connect the sewer hose to the outlet.
Just as a heads up for a new Minnie owner, make sure you check the seam at the bottom of the cabover where the outside wall meets the side to bottom trim strip as shown in the picture. I had a leak on the passenger side that resulted in some wood rot of the cabover floor. Pressing on the side wall just above the trim opened a gap between the wall and the trim strip. I keep it sealed with Dicor caulk and check for cracks in the caulk several times a year, resealing as needed. Not necessary to do a sloppy bead as shown in the picture.
