Unfortunately, once there is a build up on the sensors and is left for a longer period of time before trying to clean it off, will be very difficult to remove. To keep the sensors working properly, regular (and often) cleaning is required from day one.
Both of my motor homes were perchased new and I have never had a problem with the tank sensors in either one. However, my TT was used (if even only briefly) but was allowed to set at the bank (repossessed) for about six months and naturally, the prior owner did not clean it out prior to it being hauled off.
It took me many cleanings (see my post above) to finally get the sensors to work but once they did, they worked just fine for as long as I had the trailer.
Ron