The unit is 10 years old, no telling what could have cracked or split over time. My post was only a suggestion as something to consider. Not knowing the layout of your rig, the reference to the vent pipe is an assumption if the entertainment console backs up to the bathroom.
Here is the floorplan for the OP's Outback. The toilet (and the black tank vent space behind the toilet) are on the opposite wall from the entertainment center. In this "brochure diagram" you can see that there's a wall space in the corner behind the toilet where I'd suppose the black tank roof vent rises from the belly to the roof. The OP can confirm this by visualizing the roof to see if there's a vent stack in that location, but I'd suspect that's where the black tank vent is installed.
While on the ladder to check the black tank vent location, checking for a roof vent in the vicinity of the entertainment center will confirm or rule out a gray tank vent in the wall between the entertainment center and the shower wall.
OP:
THERE IS NO REASON TO TEAR OUT THE SHOWER SURROUND TO TRY TO LOCATE THE VENT STACK !!!!! If it isn't visible in that area on the roof, it isn't there and pulling out the shower surround will only destroy your shower.
That said, when you pull the stereo and can see behind it (in the space not visible with it installed) you can confirm whether there is any water damage to the rear wall (stains or gray color water marks) and that should tell you if there's a leak in the back part of the entertainment center caused by a leak in the wall between the shower and that cabinet.
Also, it looks like your WFCO power center is located "on the floor" of the entertainment center. It's an easy task to pull the power center out. With it out of the way, you can use a flash light to fully check the bottom of the shower wall to see if there's any water stains that would/could be a source of "mold smell in the entertainment center".
As an "added, way out there guess", there might be a Studor Mini Vent for the gray tank that's in the entertainment center rather than in the vanity cabinet. If there's no Mini Vent under your bathroom vanity, then you might find one behind the shower, in the same space as the power center. It will project up, at least to the top of the toilet height, so all you'll see behind the power center would be a black 1.5" CPVC pipe. As the first part says, this is an "added, way out there guess" and is very likely not where the mini vent is located, but with Keystone, or any other RV manufacturer, you just never know......