In addition to obviously needing to use more water when using, I mounted one of the flush valves on the side of our black tack in years past when we have a 5th wheel. Use clear silicone sealant and "snug" the mounting screws. Do not over tighten!! Ours actually sprayed all over the inside of the tank and did a pretty good job. We used it about every 2-3rd time when dumping depending on how many other folks were waiting. I really don't see how the ones mounted on the clear discharge valves can get up inside the tank very well. Another possible option is a wand type device with an on/off valve that is attached to a water hose and then brought into the bathroom via through a bath window or through the front door and stuck down the toilet before turning on. Sounds kinda gross to me.
https://www.rvsupplyparts.com/catalog/rv-holding-tank-flush-kit/valterra-no-fuss-flush-holding-tank-cleaner-a70-7164.html
