Fill your fresh water tank as Matt Colie suggests and turn off the campground water supply. You can get a good idea of how the water is being used, especially if you can see the fresh water tank.
Without a sewer hookup you can't use water like at home. Don't fill the toilet when you flush, turn off the water while you brush your teeth, take "Navy" showers, that is wet yourself down quickly, turn the water off while you wash, then turn the water on and quickly rinse the soap off.
Lots of ways to conserve water.
Edit: Could it be you have a lot of solids in your black tank?