Dennis
As Joel stated it's not necessary to flush your black tank every time you dump .
But I have some tips to help you negotiate the tanks.
First most people don't use enough water in their toilets as a rule of thumb you should fill the bowl at least half full or above .don't worry about filling the black tank too fast as the grey always fills up first .
Second it's better to have your black tank at least half full before dumping the extra water helps carry the solids away.
Third never leave the black tank dump valve open this allows the liquids to drain away leaving the solids behind creating the dreaded poop pyramid in the tank.
Fourth always dump your black before your grey this way the gray water flushed out the dump hose so its clean/eric to put away.
Now for a trick that will answer your question.
When dumping at a dump station or rest stop dump your black first once it is done running out with the hose still connected lift the hose up about two feet off the ground close to the coach and then with the black valve still open now open the gray valve and allow the grey water to run back into the black tank for several seconds then close the grey water dump and lower the hose back down this allows you to use your grey water to flush the black tank do this two or three times and then finish up by dumping your remaining grey water to flush the hose out before puting it away.
This allows you to keep things clean until you can take your time to do a good flush ( best done with a full hookup) at an RV park.
If you encounter tank order there are several tank additives on the market but over the years Ive found that this recipe from the clean tank guys works as well as any
https://kleentank.com/tank-solution-recipe/Hope this helps
Eric