Holding tank level indicator

I have had the same issue many times after dumping my Class C Winnebago. It has always been the sensor area in the tank isn't clean. I purchased a tank wand and always give the tank a thorough flush/cleaning after every trip. It is hard to do that on the road, especially with others waiting to dump behind you. Note, if you let the tank dry out, it's much harder to clean it later. Pouring detergent or Calgon works too making the tank walls slippery.
Added note: I tried the back flushing method, but only the wand with a direct jet pointed at the sensors did the job.