20-mule team borax is a water softener. Buy it at most Wal-Marts. It's a laundry detergent booster. Great stain remover for use in bathroom sink and toilet bowl.
After dumping and flushing the black tank... Close the valve... then
I put 1/2 a cup of 2x or 3x HE laundry detergent (Liquid Whisk is my choice)
and 1/2 a cup of Borax in the toilet bowl... swish it around with the toilet brush to dissolve. Fill the toilet bowl up with water - give it another swish...and let it set for an hour. Take the toilet brush and give the bowl a light scrub and flush the toilet. Fill the toilet bowl one more time with water and flush again. Done.
I put equal parts in the bathroom sink, as well. Plug the drain. Add a little water to make a paste and use a cleaning sponge to apply it to the sides of the sink. Let it set for a bit and add water to the sink and let it set for an hour. Rinse it with water. Done.
Now you've got all of that in the black tank until your next sewer dump.
If you have a washing machine... every so often just close your gray valve and do a couple loads of laundry using borax and laundry soap ...with the valve closed. Using Borax in the washing machine keeps it from smelling bad between loads.