I found this when searching for level sensors that would work on RV tanks. http://www.instructables.com/id/Building-a-Capacitive-Liquid-Sensor/
Worked well using copper tape from a hobby shop designed for stained glass working, placed about 1/8" apart on a piece of pvc. My problem came from too long of wire runs, and I'm now in the process of using ATTiny 85's on each tank sending data back via I2C since I am using an I2C LCD. Lots of work yet.
Here is a link to the system i am trying to emulate Garnet 709 SeeLevel II Tank Monitoring System, the instalation manual give a pretty good description of sensor placement requirements.