Back in the 60's Window Vans were popular. We had one it was a DIY.
Bought the Dodge van brand new. When I ordered it. Only had front seats and one rear bench seat.
Wife made drapes for the windows. I put plywood floor down with carpet.
I also out foam insulation in the walls and paneled them.
I had a rear engine heater for the kids when on the road. A wall folding cot. We traveled all over with it.
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.