Well compartment below cassette tank

NovaTab

When I removed the tank I see a well full of black water, scooped it out and see a round fitting in the bottom like plug?

Pic is with cover plate removed

Sharon_is_SAM

This may help
https://tab-rv.vanillacommunity.com/discussion/11505/cassette-toilet/p1

MuttonChops

@NovaTab the thread @Sharon_is_SAM linked has at least one additional person also commenting on that lower space having 'black water'.  From searching EU forums my WAG is:

 - 1 -  Those spaces are intended to be dry
 - 2 -  If you are getting black water then the cassette tank has not been correctly installed
           in the space and/or there is a toilet bowl to tank seal issue.

From the EU site found this reference for earlier design C402 units . . .

Date Posted: 28/2/2011
The owner's manual shows the panels (items 'e' and 'f') on the last page and page 1 explains that they are for storage of toilet fluids but notes that they are only for the 'tall model' - whatever that is!