You learn to get very conservative with water when you boondock.
Dishes get washed once a day. That water is caught in a dish pan (actually a big plastic bowl that happens to fit in my sink) and poured down the toilet. My grey tank is tiny, so I really watch how much goes into it. The black tank fills much slower.
I get a daily sponge bath. I'll also use wet wipes for this purpose. Navy shower every third day or so, or the morning I hook up and head to the next spot. Actually, the morning I hook up I'll take the time (and water) to really wash and condition my hair as well.
I stop somewhere to dump my tanks and refill the fresh water before heading to the next campsite. Sometimes I can do this at an RV resort along the way (usually $15 or so). Sometimes I need to go to a particular gas station. Sometimes I have to go out of my way to get to a ranger station or rest area. It's just part of the trip planning.