If you dump baking soda in the black tank, which by itself shouldn't be a problem, be careful what you else put in it. You could end up with a brown fizzy bubbling out of the toilet.
Something I do 2-3 times a season (starting when I de-winterize), is mix two capfuls of liquid water softener in hot water and pour it down the toilet followed by two capfuls of laundry detergent. I ran across this idea
here when we bought the camper and have used it ever since. I don’t generally mess with dumping chlorine bleach down the toilet because of the fumes, but I have in the past.
Edit: I should have mentioned I add the water softener and laundry detergent after rinsing and dumping the black the tank.
