There is a very interesting article in this months RVers Corner (rverscorner.com)
Here is an excerpt......
One of the more common complaints we hear about RVs these days is holding tank sensor failure. Although not an earth shattering problem, ..............waste water starts backing up into the toilet, sink, or shower, but your gauges show that the tank is only a quarter full. Yuck! Aside from cleaning up the disgusting mess, the problem is usually relatively easy to diagnose and correct.
The problem is either mechanical, electrical, or the result of a dirty in-tank sending unit. Here's a good, inexpensive way to find out if a dirty sending unit is indeed the culprit, and most times fix it if it is.
The next time you put your RV up between trips make sure you’ve emptied and thoroughly rinsed the holding tank. Then fill it with fresh water to nearly full, add the correct amount of an excellent quality bacteria type liquid holding tank chemical for the capacity of the tank, and fill it the rest of the way with water.
Let the RV sit as long as you can without adding anything to the tank. This will allow the bacteria and the enzymes they produce to do their job of digesting any toilet paper or organic debris that may be clinging to the sensors. Finally, before your next outing, simply empty and thoroughly rinse the tank once again. If you have allowed sufficient time (a week or so should do it) for the product to work the sensors should read correctly. If they don't then the problem is either mechanical or electrical and you'll want to ask Les to help you troubleshoot via the RVers Corner bulletin board. If all else fails you'll have to take the RV to a service center for additional diagnosis and repair.
We also recommend that this procedure be performed on an annual basis to keep both your grey and black water holding tanks, sensors, and dump valves pristine clean. At about a dollar to treat a 40 gallon holding tank, this is the least expensive insurance you'll ever buy..........
For the full text go to rverscorner.com they do have some interesting tips. hope this is usefull.
Don't take life so seriously.... it is not permanent!