The best black tank system I've seen was on a custom trailer. It was inside, and had a 12" access port on top, where you could actually go and manually scrape/poke/flush stuff off by hand that was stuck. No guesswork, no backflush... if you wanted it out, you could reach everything in the tank and get it out mechanically.
I have never used a backflush system. I don't see how that is any better than just filling the black tank from the sink or toilet. I also have seen people leaving the backflush system on, with the black tank valve closed, and poop spewing out of the rooftop exhaust vent. This is especially notable on a hot, humid day in the Texas heat.
