Well, you didn't say what type of rv or if new or used. While most black/gray tank systems are pretty much the same throughout the rv industry, you will still find some that come without the internal rinse sprayer that can really make a difference.
If you do a search on "black tank", you'll find a whole bunch of different solutions on keeping the odors down and sensors working, but when it comes right down to it, using plenty of water (during flushing and draining) will make the most difference in how successful you'll be.
If this is a used (but new to you) rv, there is no telling what the previous owner did (if anything) to help keep the sensors operational. If you are traveling, make sure the tanks are empty, add a few gallons of water to each along with some laundry or dishwashing soap. The sloshing around might dislodge what ever is clinging to the sensor probes.
If new, you obviously have not been using and draining the tanks properly. The following is my standard procedure that I have been using for many years on many different types of rv's. It has worked for me.
Since both my gray and black tank drain pipes are connected together for a single outlet (like most of the rv industry uses) I personally have installed a third slide gate to the bottom of my drain pipe along with a clear connector prior to connecting my stinky slinky. When both tanks are pretty much full, I'll dump the black tank and then close the third gate. I'll open the gray tank and then the black tank and let the soapy water from the gray flow into the black. When equalized, I'll close the gray and open the third gate...emptying the black tank once again. I'll do this two or three times all the time keeping the black tank rinse running. When the water coming out of the black tank is clear, I'll then close the black tank valve and put several gallons of fresh water back into the tank and then turn off the black tank rinse.
When traveling (which isn't all that often) I'll drain both tank (using the method above) and add several gallons of water to both tanks. I'll then add a cup full of liquid Oxo-Clean laundry detergent to each tank along with a cap full of liquid Calgon water softener. After reaching my destination, I'll drain both tanks and put a couple of gallons of fresh water back into the black tank.
This works for me, but there are many other procedures that other's use that have the same results....some use chemicals, some use only water. The common denominator is WATER. Use lots of it when flushing and draining....never leave black tank valve open.
Ron, Sandie, and Lilly
2013 Tiffin Phaeton 42LH
2011 GMC Terrain SLT-2