I had a couple of old 6 gallon red jugs left from back in the days we had to keep water available if our storage tank went dry. They aren’t potable water rated of course so we carry a few gallons of purchased water for drinking and cooking. That is for the sink washing and our cleanups. I see plenty of rated jugs available so I might someday change my fresh water jug and just use these for the grey water.
I use the Scepter brand military surplus containers (They're often called "Jerry Cans"). They are expensive, but they are built like tanks. My newest sink setup will consist of a 5 gallon can for freshwater and a 2 1/2 for the gray water.
Another nice thing about the military containers is that they come in black. You can set them out in the sun to absorb heat during the day and use them for washing, or to cut down on your fuel usage to further heat what for cooking, etc.
I have two of those, too, but they are only for serious boondocking on one particular trip each fall. I cannot maneuver a full one. We normally use the square one-gallon containers from the dollar store. We keep one handy up front. There is space for about 10 in the rear, but three is the norm. We don't use much water.