We did five days easily with three adults (and three visitors who stayed one night with us) at a state park where we had electric hookup only and on a different trip five days with just the two of us with only electric.
Take quick showers - get wet - soap up and shampoo with water off - then rinse off!
We generally use the campground facilities during the day and save the black tank for after dark.
When we don't have water hookup, we do our dishes outside and use our old camp stove to get the dish water hot.
In this photo, notice the tan dish tubs upside down drying out on the picnic table:
I also have 200 feet of hose, a water thief and two 10 gallon water jugs with wheels when we need to fill up fresh water tank.