Many RVs have multiple waste water tanks. The second waste water tank is typically called the "galley" tank.
I suppose there could be a second black tank as well but don't personally know of any RVs that do.
Other tank types can't be repurposed as a waste water tank because the warning level is reversed for waste tanks: most types warn below a low level, waste tanks warn above a high high level.
A more generic change would allow the warning level to be set with a higher/lower flag for each tank