Robert, How did you rig up your water tank and how are you installing a wastewater holding tank?
For my mobile wet-plate work, I have a cargo trailer that I have converted with an air conditioner, a vent like yours with ruby lith for a safelight and insulation on the roof. I use an office chair because the ceiling is too low for me to stand in. For a sink, I bought a cheap, plastic laundry sink. All but a couple of sticks of wood were surplus lumber I had laying around and I scooped up a set of metal shelves left on the side of a road to store trays and other paraphernalia. I painted the floor and walls with a can of gray reject paint.
I usually drain the water into a bucket and have to admit that if I'm on a parking lot I just let the spent developer and wash water run onto the pavement. For water, I either buy a 2-gallon plastic container with one of those spigots but I'm about to buy a 5-gallon container used for camping.