Regular use will help.
It would appear that you did not flush after your last dump. Otherwise, the crud would have not had a chance to dry.
If you are continuing to use the tank, flushing should not be needed every time, but if you are going to leave the tank dry for any length of time, you should flush it, for the tank and the spray wand benefit.
How to save the labour charges? Fill the black tank, with water, add dish detergent, and let it soak for a day or so. That should work.
"LOVE and LOSS, are two of the greatest emotions one can experience. -- I went to school to learn about "WHAT GOES UP MUST COME DOWN" but I had to live my life to learn the lesson of: 'WITH LOVE THERE WILL BE SORROW'."
David Stewart. (after loosing my NAVIGATOR)