After you dump tomorrow, add water to the tank and a couple cups full of bleach. Then drive to your next destination and dump once you get there. The bleach will do wonders!
We use our camper at home all the time. I dump the black tank once a week (I have the ability to use my septic tank at home). If I go much longer than a week, every time we flush, and the valve opens in the toilet, that quick moment to flush makes the entire camper smell awful! The best cure is a good road trip with water and bleach in the tank, and then dump right away.
The driving and sloshing will be the best cleaner tool you can get. Dump immediately, rinse.