Most likely it is for an option that they never offered, but, they got a "good deal" on the boards and panels from the manufacturer
It may be something that they planned on offering, but, it proved too costly in manufacture or it may be for another model that does have twin gray tanks (gray; shower and bathroom sink, galley; kitchen sink). Only CruiserRV would know what they intended it for...my '08 210WBS had only those meters that were used, no extras, but, I do know that the manufacturers order those boards and they are available in multiple configurations.
Maybe, somebody has an "Aux" meter that is actually attached to something? Would be interesting to know if CruiserRV actually has a model that needs that extra input...