Fresh water is what you use yo cook and drink and bath with.
Gray water is waste water.
Blank water is waste water from the head(toilet).
So do not think you mean mix FW with Gray before use.
BLK and gray can be mixed.
Do you have an inverter?
If using it a lot while driving down the road. This Gould be a big draw that the engine altanator can not charge fast enough.
My wife used to use it a lot watching TV until the KD died.