Showering and strong odor from black tank.......:H
The 2 are not connected.
Shower drains to grey tank.
Could be 'stuff' in shower 'P' trap that is getting stirred up while using shower.
Could be the air admittance valve under bathroom sink that is getting pulled open while shower water is draining down to grey tank (grey tank odors can stick as bad as black tank)
Could be smelly Hot water while using shower........which would also smell when using any hot water.
Just don't think smell is from black tank while using shower (unless someone is using toilet at same time)