its very common for these tanks to get crud on the sensors and not function correctly.. and as many people here have their own idea how to solve this..
here are my thoughts.. both to keep the sensors clean and to stop the odor.. we personally use OXYCLEAN in the black and gray tanks... once we dump the tanks we had back 2-3 gallons of fresh water and a couple of cups of oxyclean... this has worked for us over the years and we now spend 4 months a year out in our RV..
On the way home for storage we stop 200 or so miles from home and dump the tanks, and recharge them now with about 10 gallons of water.. we add the oxyclean and we won't use the tanks again... at the last stop before we get home we dump the tanks another time.. we have a clear extension on the end of the hose and you'd be surprised how, with no use, the dirt and stuff that now comes out of the tanks from sloshing around for 200 miles.. once we'd drained and flushed the tanks we charge them again, take the unit home and store it... if its stored over a cold winter we drain and leave the valves open for the water to evaporate out of the tank to prevent freezing..
Of course this is how we do it... its simple, works for us, we have no odors and we don't spend a lot of money on chemical and the wife always has oxyclean on the unit for doing wash... I'm sure that there are lots of other methods, and I'm sure they work for those RVers... to each his own so to speak..
Retired Business Owner, Re-manufacturing HD Clutches, Brake Shoes, Air Compressors, Sales & Installation of PacBrake and other Industrial Friction