Well here you go, more research LOL! A nice long article about the different ways to measure fluid level in a tank (more for industrial processes though but nevertheless a good overview):
A Dozen Ways to Measure Fluid Level and How They Work | Sensors
Here is a DIY capacitive level sensor project:
http://www.instructables.com/id/Buil...nsor/?ALLSTEPS
Since it is using an Arduino microcontroller it should be pretty easy to have the Arduino ground the 3 tank probe pins to light up the levels on the RV's panel (assuming that the "bottom" probe is connected to ground and the fluid in the tank just grounds the probe when in contact).
Hmmm
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