My father in-law bought a used 5th wheel that a guy had put a big basket/cage on the rear. It was well done when it came to mounting to the frame. Held a lot of weight.
Father in-law found an old 55 gallon drum. Cleaned and sanitized it for water. Mounted it in the basket on the back. Then he spliced into the water line from the trailer tank with a valve. Got some water line and would run it back to the 55Gal drum. Used it first, then used the trailer tank last.
As for the extra grey water issue, he would take 2 5 gal buckets and drain water and pour it down the grey water disposals at the camp ground.
As for extra black water? Rule was, use out house during the day, trailer at night.