I prefer bio cleaners. I've used them in the past with great results. I don't know exactly what they are made of, but I think it might be enzymes that break down the waste. I bought an liquid 4 oz. bottle at at RV store in Texas and poured it in my empty black tank. At the time my sensors were all reading full not matter how much flushing I did of the tank. After I poured that bottle in the tank I used the tank as normal for a week while we sat in an RV park in Oklahoma City. Before leaving for Branson I dumped the tank and discovered that my sensors were reading empty and after I did my normal fill/flush with clean water my tank contents passing my clear section of pipe was clean. I was very impressed. Go to your local RV store and see if they have liquid enzyme tank cleaner and give it a try.

I've also used Thetford Tank Level Gauge cleaner...Before pulling out of an RV park I dump the tanks, then put 1/4 tank of water in the tanks and add the cleaner. I then drive to the next park and use the tank as normal until full before dumping.

I found the internal tank sprayer to be almost useless...I could let that thing run for an hour with the waste gate valve open and still have sensors that read incorrectly. Not to mention a waste of my time to monitor the water. I've found the additives are much simpler to use.