This. I have seen SO MANY PEOPLE have issues with solids in the black tank. And if you don't get it out, it decays, stinks, and turns into an even bigger solid.
While it's true that for a stationary septic system you don't need to add anything to break down what goes down, black tanks in an RV are NOT the same. There's no two-stage tank to let solids settle out and degrade over time. The biggest RV tanks I've seen are good for a week, a week and a half at most, then they NEED to be dumped because they're full. If you've got some sizeable solids in there, and they get stuck together, you can EASILY clog up your drain tube. This leads to either the RV going to a shop, or a mobile guy coming out to fix it (because I really don't feel like messing with my own crap), and it's not always as simple as sticking a rotor rooter up the drain pipe - sometimes things have to be disassembled, and that's just a smelly mess.
Far better to put something in the tank that breaks it down and keeps things flowing.