We were camping with our 2 adults daughters along last month, in Yellowstone with no hookups. We were there for several days, in our Sport with its 20 gallon fresh tank and 22 gallon gray tank.
We explained to the daughters to turn off the water while "washing" to conserve water. No one ran out of hot water, even with washing hair. We did have to empty the gray tank daily and top off the fresh tank daily but this was really not a big deal. Actually we found that emptying the gray right after someone showered was the best time, as the water was not every sitting in the tank very long. Draining into the blue boy was fairly simple and it was easy to dump out.
But the hot water heater works very well.