The water in the heater gets used if you take showers or anything else using warm water.
My camper goes to Burning Man (hence my username) where absolutely zero grey water, trash, or anything goes on the ground.
Sometimes it goes to races or car shows where you can’t dump anything in the ground.
It’s going to the Bonneville Salt Flats where you can’t dump on the ground. In fact you have to park on a tarp so you don’t so much as leak a drop of oil.
Sometimes it’s even a portable guest house for visiting friends
I’m a little surprised I’m that much the odd truck camper user. I almost never go to a campground.
My primary reason for owning it is the heavy trailer pulling ability.