Every manufacturer has different plumbing set ups, some even differ between floor plans. Shower draining into black tank would be unusual.
I've had rear kitchen 5ers that had 2 grey tanks, and one with two grey tanks drained the vanity sink into the black; to add more liquid to the tank to promote draining.
About the only way to be sure where the shower drains; if you can't see a connection;, would be to run water in the shower with the black valve open and see if you can feel the water running down the sewer hose. Or buy a clear plastic sewer adaptor.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.