I don't bother using RidEx. What works to keep the solids dissolved for me is to squirt some Dawn hand dish detergent in the black tank. If it removes grease from a pan, it will do it to gunk floating in the black tank.
I'm cheap. I don't want to throw money down the toilet (literally), so I just use something inexpensive.
Now, if I'm dumping into a septic system, for the sake of the system, I'd toss the bacteria cakes in, but if I'm dumping into a town sewer, I'll save the $7-$20.
Oh, I use plenty of water as well. I like dumping the tank, filling it up again, then dumping again. That way it is as clean as possible for the road. I also add some hand dish soap and about a gallon of water as an insurance against dried up pooprock.