I have a Pop Up tent camper. Our black tank is less than 3 gallons on the porta potty. Wife and I, 2 days is easy if we stay out of restaurants. (This assumes no other restroom near. If I can walk a quarter mile thru the day, only dump at end of 3 night stay. Or in the toilet I'm walking to if staying longer.
Gray water. The inside sink is only used for sponge baths (Waterless shampoo and no rinse soap about qt water) and teeth brushing. The drain hose goes into a large laundry soap jug.
Dishes is the big water hog. I set up a sink outside, drain into 7 gallon jug. Use 2 pans. 1 for hot soapy water. Rinse with a small stream of water, and catch it in 2nd dish pan. When done dump the wash water, (Dad kept a bucket, would use that to fill the bowl before number 2. Rest of time just enough water in bowl to cover the seal) Save that rinse water. Next time you do dishes heat a pot of water, (Where does the water that you drain out of pipes to find hot go?) add that and soap to do dishes.
