I believe that in the side bath model at least, when the shower is used, the wax ring under the black water tank functions to contain the water in the shower pan (floor) and prevents the water from coming out the hole in the case that water accumulates because the drain pump is off or clogged (or the camper is bow high). I suspect that is you have the same type of arrangement, and leave the drain pump off while running water in the shower, that you will see water come out the black water drain pipe hole if the wax ring has deteriorated.
As a secondary function, it blocks insects, snakes, etc. I am sure you have to remove the entire commode and tank to replace it, a PIA. Its a standard commode wax ring, although not used to seal in the same way (at the base of the commode).
You will have to decide if it needs to be replaced based on you needs. You could seal up the hole from underneath to block critters, but if water from the shower pan gets on top of the sealing, it will be in the floor (under the shower pan) and rot would no doubt follow. If you never use the shower, then thats not an issue (unless maybe the commode starts to leak into the shower pan).
At least thats how I understand the construction. Perhaps instead of just a cutout for the black water drain pipe, the shower pan should have a sealed "funnel" through the floor with the drain pipe run through it, so that water could
escape the shower pan to the ground.