I have heard countless descriptions of various methods of cleaning the tank to keep the wires clean - ranging from ice cubes to detergent. Unfortunately, most of the time the sensors will get clogged again and again, forcing you to repeat the cleaning routine.
An alternative is to just replace the sensor you have with a better one. One that many have claimed to work well is the SeeLevel sensor. This is specifically designed for RVs, and the best part is that it never gets clogged, as it is located on the outside of the tank. Many have been pleased with the results of this sensor, and it seems to work fantastically in this field. The only complaints that I have heard thus far is that it is pricey, and it isn't waterproof.
If either of these problems concern you, you could also try this water level sensor. It is considerably cheaper, and is contained in a watertight enclosure.