This is my operations for catching gray water. I use a couple of the AQUA-TAINER 7.5 GALLON totes to catch the sink water. I rigged up a short water hose that pushes down into the tote. When one gets full then I move the hose to the empty tote and haul off the full one to the camp bath room. I have same connections for the cassette shower combo you can see here under the fridge vents...
The black water of the cassette potti has its own 6-gallon tank with wheels.
I use a two wheeled folding cart to haul the totes with and bring back fresh water.
It is amazing how much water one has to tote off during one day... Wears me out sometimes...
Roy Ken